[jextract] Improve CDecl lowering

* Newly introduce `lowerResult()` (splitted from `lowerParameter()`
  because result lowering can be differnt from parameter lowering.

* Remove `ConversionStep.init(cdeclToSwift:)/.init(swiftToCdecl:)`.
  Instead embed the logic in `lowerParameter()`/`lowerResult()` so that
  we can implement type lowering and the actual conversion at the same
  place.

* Support 'String', 'Void' and '() -> Void) types

* Implement getter / setter lowering

* Fix conversion for tuple returns (see: `lowerTupleReturns()` test)

* Reference types are now returned indirectly, as the logic are being
  unifed. Thanks to this we don't need manual `retain()` before
  returning instances, which wasn't implemented correctly.

Author: rintaro

Sources/JExtractSwift/CDeclLowering/CDeclConversions.swift

@@ -24,10 +24,22 @@ extension CType {
   init(cdeclType: SwiftType) throws {
     switch cdeclType {
     case .nominal(let nominalType):
-      if let knownType = nominalType.nominalTypeDecl.knownStandardLibraryType,
-        let primitiveCType = knownType.primitiveCType {
-        self = primitiveCType
-        return
+      if let knownType = nominalType.nominalTypeDecl.knownStandardLibraryType {
+        if let primitiveCType = knownType.primitiveCType {
+          self = primitiveCType
+          return
+        }
+
+        switch knownType {
+        case .unsafePointer where nominalType.genericArguments?.count == 1:
+          self = .pointer(.qualified(const: true, volatile: false, type: try CType(cdeclType: nominalType.genericArguments![0])))
+          return
+        case .unsafeMutablePointer where nominalType.genericArguments?.count == 1:
+          self = .pointer(try CType(cdeclType: nominalType.genericArguments![0]))
+          return
+        default:
+          break
+        }
       }
 
       throw CDeclToCLoweringError.invalidNominalType(nominalType.nominalTypeDecl)
@@ -109,7 +121,8 @@ extension KnownStandardLibraryType {
     case .unsafeRawPointer: .pointer(
       .qualified(const: true, volatile: false, type: .void)
     )
-    case .unsafePointer, .unsafeMutablePointer, .unsafeBufferPointer, .unsafeMutableBufferPointer:
+    case .void: .void
+    case .unsafePointer, .unsafeMutablePointer, .unsafeBufferPointer, .unsafeMutableBufferPointer, .string:
        nil
     }
   }