Auto merge of #28642 - petrochenkov:name3, r=nrc

This PR removes random remaining `Ident`s outside of libsyntax and performs general cleanup
In particular, interfaces of `Name` and `Ident` are tidied up, `Name`s and `Ident`s being small `Copy` aggregates are always passed to functions by value, and `Ident`s are never used as keys in maps, because `Ident` comparisons are tricky.

Although this PR closes https://github.com/rust-lang/rust/issues/6993 there's still work related to it:
- `Name` can be made `NonZero` to compress numerous `Option<Name>`s and `Option<Ident>`s but it requires const unsafe functions.
- Implementation of `PartialEq` on `Ident` should be eliminated and replaced with explicit hygienic, non-hygienic or member-wise comparisons.
- Finally, large parts of AST can potentially be converted to `Name`s in the same way as HIR to clearly separate identifiers used in hygienic and non-hygienic contexts.

r? @nrc

src/test/auxiliary/lint_plugin_test.rs

@@ -35,7 +35,7 @@ impl LintPass for Pass {
 
 impl EarlyLintPass for Pass {
     fn check_item(&mut self, cx: &EarlyContext, it: &ast::Item) {
-        if it.ident.name == "lintme" {
+        if it.ident.name.as_str() == "lintme" {
             cx.span_lint(TEST_LINT, it.span, "item is named 'lintme'");
         }
     }

src/test/auxiliary/procedural_mbe_matching.rs

@@ -33,7 +33,7 @@ fn expand_mbe_matches(cx: &mut ExtCtxt, sp: Span, args: &[TokenTree])
 
     let mac_expr = match TokenTree::parse(cx, &mbe_matcher[..], args) {
         Success(map) => {
-            match (&*map[&str_to_ident("matched")], &*map[&str_to_ident("pat")]) {
+            match (&*map[&str_to_ident("matched").name], &*map[&str_to_ident("pat").name]) {
                 (&MatchedNonterminal(NtExpr(ref matched_expr)),
                  &MatchedSeq(ref pats, seq_sp)) => {
                     let pats: Vec<P<Pat>> = pats.iter().map(|pat_nt|