Move some Map methods onto TyCtxt.

The end goal is to eliminate `Map` altogether.

I added a `hir_` prefix to all of them, that seemed simplest. The
exceptions are `module_items` which became `hir_module_free_items` because
there was already a `hir_module_items`, and `items` which became
`hir_free_items` for consistency with `hir_module_free_items`.

Author: nnethercote

examples/rustc-driver-interacting-with-the-ast.rs

@@ -75,7 +75,7 @@ impl rustc_driver::Callbacks for MyCallbacks {
             let item = hir_krate.item(id);
             // Use pattern-matching to find a specific node inside the main function.
             if let rustc_hir::ItemKind::Fn(_, _, body_id) = item.kind {
-                let expr = &tcx.hir().body(body_id).value;
+                let expr = &tcx.hir_body(body_id).value;
                 if let rustc_hir::ExprKind::Block(block, _) = expr.kind {
                     if let rustc_hir::StmtKind::Let(let_stmt) = block.stmts[0].kind {
                         if let Some(expr) = let_stmt.init {