`scan_escape` currently has a fast path (for when the first char isn't
'\\') and a slow path.

This commit changes `scan_escape` so it only handles the slow path, i.e.
the actual escaping code. The fast path is inlined into the two call
sites.

This change makes the code faster, because there is no function call
overhead on the fast path. (`scan_escape` is a big function and doesn't
get inlined.)

This change also improves readability, because it removes a bunch of
mode checks on the the fast paths.

and optimize the iterator returned by `tokenize().

This improves lexer performance by 35%

In the lexer, consider emojis to be valid identifiers and reject
them later to avoid knock down parse errors.

The PR had some unforseen perf regressions that are not as easy to find.
Revert the PR for now.

This reverts commit 6ae8912a, reversing
changes made to 86d6d2b7.

Co-authored-by: NNiko Matsakis <niko@alum.mit.edu>

This commit denies any identifiers immediately followed by
one of three tokens `"`, `'` or `#`, which is stricter than
the requirements of RFC 3101 but may be necessary according
to the discussion at [Zulip].

[Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/268952-edition-2021/topic/reserved.20prefixes/near/238470099

Changes `librustc_X` to `rustc_X`, only in documentation comments.
Plain code comments are left unchanged.

Also fix incorrect file paths.

Fixed it by inlining it in the two places where the count is used and simplified the logic there.

Also add an Oxford comma while we're editing that line.

Lexer now discerns between regular comments and doc comments, so use that.
The change only affects the choice of reported errors.

StringReader is an intornal abstraction which at the moment changes a
lot, so these unit tests cause quite a bit of friction.

Moving them to rustc_lexer and more ingerated-testing style should
make them much less annoying, hopefully without decreasing their
usefulness much.

Note that coloncolon tests are removed (it's unclear what those are
testing).

\r\n tests are removed as well, as we normalize line endings even
before lexing.