Create hir-place-target.md

Author: nikomatsakis

hir-place-target.md

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+We are going to be refactoring [HIR places] to capture sufficient detail for us to use them to describe the places that a closure captures. 
+
+The expected target structure looks something like this:
+
+```rust
+/// A reference to a particular place that appears in the source code
+struct PlaceReference<'tcx> {
+    /// the place being referenced
+    place: Place<'tcx>,
+    
+    /// hir-id of source expression or pattern
+    hir_id: HirId,
+    
+    // Maybe no Span, since they're being removed from the Hir
+}
+
+/// An expression that refers to some place in memory, similar
+/// to MIR places
+struct Place<'tcx> {
+    /// start of the place expression, typically a local variable
+    base: PlaceBase,
+    
+    /// projections select parts of the base expression; e.g., 
+    /// in the place expression `a.b.c`, `b` and `c` are projections
+    projections: Vec<Projection<'tcx>>,
+}
+
+/// *Projections* select parts of the base expression; e.g., 
+/// in the place expression `a.b.c`, `b` and `c` are projections
+struct Projection<'tcx> {
+    /// Type of the projection thus far.
+    ty: Ty<'tcx>,
+    
+    /// 
+    kind: ProjectionKind,
+}
+
+/// Kinds of projections
+enum ProjectionKind {
+    /// `*B`, where `B` is the base expression
+    Deref,
+    
+    /// `B.F` where `B` is the base expression and `F` is
+    /// the field. The field is identified by which variant
+    /// it appears in along with a field index. The variant
+    /// is used for enums.
+    Field(Field, VariantIdx),
+    
+    /// Some index like `B[x]`, where `B` is the base
+    /// expression. We don't preserve the index `x` because
+    /// we won't need it.
+    Index,
+    
+    /// A subslice covering a range of values like `B[x..y]`.
+    Subslice,
+}
+```