internal/pkgbits: indent productions and hoist some types up

The types being hoisted are those which cannot be referenced; that is,
where Ref[T] is illegal. These are most clearly owned by pkgbits. The
types which follow are those which can be referenced.

Referenceable types are more hazy due to the reference mechanism of UIR
- sections. These are a detail of the UIR file format and are surfaced
directly to importers.

I suspect that pkgbits would benefit from a reference mechanism not
dependent on sections. This would permit us to push down many types
from the noder into pkgbits, reducing the interface surface without
giving up deduplication.

Change-Id: Ifaf5cd9de20c767ad0941413385b308d628aac6c
Reviewed-on: https://go-review.googlesource.com/c/go/+/674635
Auto-Submit: Mark Freeman <mark@golang.org>
TryBot-Bypass: Mark Freeman <mark@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>

Author: Mark Freeman

src/internal/pkgbits/doc.go

@@ -8,88 +8,88 @@ package pkgbits.
 
 The most basic primitives are laid out as below.
 
-Bool    = [ Sync ] byte .
-Int64   = [ Sync ] zvarint .
-Uint64  = [ Sync ] uvarint .
+       Bool    = [ Sync ] byte .
+       Int64   = [ Sync ] zvarint .
+       Uint64  = [ Sync ] uvarint .
 
-zvarint = (* a zig-zag encoded signed variable-width integer *) .
-uvarint = (* an unsigned variable-width integer *) .
+       zvarint = (* a zig-zag encoded signed variable-width integer *) .
+       uvarint = (* an unsigned variable-width integer *) .
+
+# References
+References specify the location of a value. While the representation here is
+fixed, the interpretation of a reference is left to other packages.
+
+       Ref[T] = [ Sync ] Uint64 . // points to a value of type T
+
+# Markers
+Markers provide a mechanism for asserting that encoders and decoders
+are synchronized. If an unexpected marker is found, decoding panics.
+
+       Sync = uvarint          // indicates what should follow if synchronized
+              WriterPCs
+              .
+
+A marker also records a configurable number of program counters (PCs) during
+encoding to assist with debugging.
+
+       WriterPCs = uvarint     // the number of PCs that follow
+                   { uvarint } // the PCs
+                   .
+
+Note that markers are always defined using terminals — they never contain a
+marker themselves.
 
 # Strings
 A string is a series of bytes.
 
-// TODO(markfreeman): Does this need a marker?
-String    = { byte } .
+       // TODO(markfreeman): Does this need a marker?
+       String    = { byte } .
 
 Strings are typically not encoded directly. Rather, they are deduplicated
 during encoding and referenced where needed; this process is called interning.
 
-StringRef = [ Sync ] Ref[String] .
+       StringRef = [ Sync ] Ref[String] .
 
 Note that StringRef is *not* equivalent to Ref[String] due to the extra marker.
 
-# References
-References specify the location of a value. While the representation here is
-fixed, the interpretation of a reference is left to other packages.
-
-Ref[T] = [ Sync ] Uint64 . // points to a value of type T
-
 # Slices
 Slices are a convenience for encoding a series of values of the same type.
 
-// TODO(markfreeman): Does this need a marker?
-Slice[T] = Uint64 // the number of values in the slice
-           { T }  // the values
-           .
+       // TODO(markfreeman): Does this need a marker?
+       Slice[T]  = Uint64 // the number of values in the slice
+                   { T }  // the values
+                   .
 
 # Constants
 Constants appear as defined via the package constant.
 
-Constant = [ Sync ]
-           Bool        // whether the constant is a complex number
-           Scalar      // the real part
-           [ Scalar ]  // if complex, the imaginary part
-           .
+       Constant = [ Sync ]
+                  Bool       // whether the constant is a complex number
+                  Scalar     // the real part
+                  [ Scalar ] // if complex, the imaginary part
+                  .
 
 A scalar represents a value using one of several potential formats. The exact
 format and interpretation is distinguished by a code preceding the value.
 
-Scalar   = [ Sync ]
-           Uint64      // the code indicating the type of Val
-           Val
-           .
-
-Val      = Bool
-         | Int64
-         | StringRef
-         | Term        // big integer
-         | Term Term   // big ratio, numerator / denominator
-         | BigBytes    // big float, precision 512
-           .
+       Scalar   = [ Sync ]
+                  Uint64      // the code indicating the type of Val
+                  Val
+                  .
 
-Term     = BigBytes
-           Bool        // whether the term is negative
-           .
+       Val      = Bool
+                | Int64
+                | StringRef
+                | Term        // big integer
+                | Term Term   // big ratio, numerator / denominator
+                | BigBytes    // big float, precision 512
+                .
 
-BigBytes = StringRef . // bytes of a big value
-
-# Markers
-Markers provide a mechanism for asserting that encoders and decoders are
-synchronized. If an unexpected marker is found, decoding panics.
+       Term     = BigBytes
+                  Bool        // whether the term is negative
+                  .
 
-Sync = uvarint          // indicates what should follow if synchronized
-       WriterPCs
-       .
-
-A marker also records a configurable number of program counters (PCs) during
-encoding to assist with debugging.
-
-WriterPCs = uvarint     // the number of PCs that follow
-            { uvarint } // the PCs
-            .
-
-Note that markers are always defined using terminals — they never contain a
-marker themselves.
+       BigBytes = StringRef . // bytes of a big value
 */
 
 // Package pkgbits implements low-level coding abstractions for Unified IR's