It's a less powerful duplicate of `SyntaxExtension::NormalTT`

…some Mode, and more comments

Changes:
````
travis: disable rls integration test.
rustup rust-lang#61669
Add OUTER_EXPN_INFO lint
````

This makes parsing faster, particularly for code with large constants,
for two reasons:
- it skips all the keyword comparisons for literals;
- it replaces the unnecessary `parse_literal_maybe_minus` call with
  `parse_lit`, avoiding an unnecessary allocation via `mk_expr`.

Replaced linear token counting macros with optimized implementation

There are currently two distinct token-counting macros in the source. Both implement the trivial algorithm, with linear complexity. They may or may not be adequate for their use case, but considering that other people are probably going to copy and paste them whenever they need a token-counting macro, I replaced them with an optimized implementation with logarithmic complexity.

…ddyb

Const qualification comments

I extracted some const-qualif knowledge from @eddyb. This is my attempt to turn that into comments.

Cc @oli-obk 	@eddyb

Fix issues with const argument inference

Fixes rust-lang#60724.
Fixes rust-lang#61346.

r? @eddyb

Remove some legacy proc macro flavors

Namely
- `IdentTT` (`foo! ident { ... }`). Can be replaced with `foo! { ident ... }` or something similar.
- `MultiDecorator`. Can be replaced by `MultiModifier` (aka `LegacyAttr` after renaming).
- `DeclMacro`. It was a less powerful duplicate of `NormalTT` (aka `LegacyBang` after renaming) and can be replaced by it.

Stuff like this slows down any attempts to refactor the expansion infra, so it's desirable to retire it already.
I'm not sure whether a lang team decision is necessary, but would be nice to land this sooner because I have some further work in this area scheduled.

The documentation commit (rust-lang@a9397fd) describes how the remaining variants are different from each other and shows that there's actually some system behind them.

The last commit renames variants of `SyntaxExtension` in more systematic way.
- `ProcMacro` -> `Bang`
- `NormalTT` -> `LegacyBang`
- `AttrProcMacro` -> `Attr`
- `MultiModifier` -> `LegacyAttr`
- `ProcMacroDerive` -> `Derive`
- `BuiltinDerive` -> `LegacyDerive`

All the `Legacy*` variants are AST-based, as opposed to "modern" token-based variants.

…pr, r=petrochenkov

Special-case literals in `parse_bottom_expr`.

This makes parsing faster, particularly for code with large constants,
for two reasons:
- it skips all the keyword comparisons for literals;
- it skips the allocation done by the `mk_expr` call in
  `parse_literal_maybe_minus`.

r? @petrochenkov

…hearth

submodules: update clippy from 71be6f6 to c0dbd34

Changes:
````
travis: disable rls integration test.
rustup rust-lang#61669
Add OUTER_EXPN_INFO lint
````

Should fix clippy toolstate