Speed up Parser::expected_token_types.

The parser pushes a `TokenType` to `Parser::expected_token_types` on
every call to the various `check`/`eat` methods, and clears it on every
call to `bump`. Some of those `TokenType` values are full tokens that
require cloning and dropping. This is a *lot* of work for something
that is only used in error messages and it accounts for a significant
fraction of parsing execution time.

This commit overhauls `TokenType` so that `Parser::expected_token_types`
can be implemented as a bitset. This requires changing `TokenType` to a
C-style parameterless enum, and adding `TokenTypeSet` which uses a
`u128` for the bits. (The new `TokenType` has 105 variants.)

The new types `ExpTokenPair` and `ExpKeywordPair` are now arguments to
the `check`/`eat` methods. This is for maximum speed. The elements in
the pairs are always statically known; e.g. a
`token::BinOp(token::Star)` is always paired with a `TokenType::Star`.
So we now compute `TokenType`s in advance and pass them in to
`check`/`eat` rather than the current approach of constructing them on
insertion into `expected_token_types`.

Values of these pair types can be produced by the new `exp!` macro,
which is used at every `check`/`eat` call site. The macro is for
convenience, allowing any pair to be generated from a single identifier.

The ident/keyword filtering in `expected_one_of_not_found` is no longer
necessary. It was there to account for some sloppiness in
`TokenKind`/`TokenType` comparisons.

The existing `TokenType` is moved to a new file `token_type.rs`, and all
its new infrastructure is added to that file. There is more boilerplate
code than I would like, but I can't see how to make it shorter.

compiler/rustc_builtin_macros/src/cfg.rs

@@ -6,6 +6,7 @@ use rustc_ast::token;
 use rustc_ast::tokenstream::TokenStream;
 use rustc_errors::PResult;
 use rustc_expand::base::{DummyResult, ExpandResult, ExtCtxt, MacEager, MacroExpanderResult};
+use rustc_parse::exp;
 use rustc_span::Span;
 use {rustc_ast as ast, rustc_attr_parsing as attr};
 
@@ -48,9 +49,9 @@ fn parse_cfg<'a>(
 
     let cfg = p.parse_meta_item_inner()?;
 
-    let _ = p.eat(&token::Comma);
+    let _ = p.eat(exp!(Comma));
 
-    if !p.eat(&token::Eof) {
+    if !p.eat(exp!(Eof)) {
         return Err(cx.dcx().create_err(errors::OneCfgPattern { span }));
     }