Overhaul the intravisit::Map trait.

First of all, note that `Map` has three different relevant meanings.
- The `intravisit::Map` trait.
- The `map::Map` struct.
- The `NestedFilter::Map` associated type.

The `intravisit::Map` trait is impl'd twice.
- For `!`, where the methods are all unreachable.
- For `map::Map`, which gets HIR stuff from the `TyCtxt`.

As part of getting rid of `map::Map`, this commit changes `impl
intravisit::Map for map::Map` to `impl intravisit::Map for TyCtxt`. It's
fairly straightforward except various things are renamed, because the
existing names would no longer have made sense.

- `trait intravisit::Map` becomes `trait intravisit::HirTyCtxt`, so named
  because it gets some HIR stuff from a `TyCtxt`.
- `NestedFilter::Map` assoc type becomes `NestedFilter::MaybeTyCtxt`,
  because it's always `!` or `TyCtxt`.
- `Visitor::nested_visit_map` becomes `Visitor::maybe_tcx`.

I deliberately made the new trait and associated type names different to
avoid the old `type Map: Map` situation, which I found confusing. We now
have `type MaybeTyCtxt: HirTyCtxt`.

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -2410,7 +2410,6 @@ pub(crate) fn provide(providers: &mut Providers) {
 /// use a different method for pretty-printing. Ideally this function
 /// should only ever be used as a fallback.
 pub fn rendered_const<'tcx>(tcx: TyCtxt<'tcx>, body: &hir::Body<'_>, def_id: LocalDefId) -> String {
-    let hir = tcx.hir();
     let value = body.value;
 
     #[derive(PartialEq, Eq)]
@@ -2467,7 +2466,7 @@ pub fn rendered_const<'tcx>(tcx: TyCtxt<'tcx>, body: &hir::Body<'_>, def_id: Loc
 
         // Otherwise we prefer pretty-printing to get rid of extraneous whitespace, comments and
         // other formatting artifacts.
-        Literal | Simple => id_to_string(&hir, body.id().hir_id),
+        Literal | Simple => id_to_string(&tcx, body.id().hir_id),
 
         // FIXME: Omit the curly braces if the enclosing expression is an array literal
         //        with a repeated element (an `ExprKind::Repeat`) as in such case it