rename the hir chapter to The HIR

Author: nikomatsakis

src/SUMMARY.md

@@ -8,7 +8,7 @@
 - [The parser](./the-parser.md)
 - [Macro expansion](./macro-expansion.md)
 - [Name resolution](./name-resolution.md)
-- [HIR lowering](./hir-lowering.md)
+- [The HIR (High-level IR)](./hir.md)
 - [The `ty` module: representing types](./ty.md)
 - [Type inference](./type-inference.md)
 - [Trait resolution](./trait-resolution.md)

src/hir-lowering.md renamed to src/hir.md

@@ -1,4 +1,4 @@
-# HIR lowering
+# The HIR
 
 The HIR -- "High-level IR" -- is the primary IR used in most of
 rustc. It is a desugared version of the "abstract syntax tree" (AST)
@@ -116,4 +116,4 @@ associated with an **owner**, which is typically some kind of item
 (e.g., a `fn()` or `const`), but could also be a closure expression
 (e.g., `|x, y| x + y`). You can use the HIR map to find the body
 associated with a given def-id (`maybe_body_owned_by()`) or to find
-the owner of a body (`body_owner_def_id()`).
+the owner of a body (`body_owner_def_id()`).