`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.

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.

Unified `unescape_{char,byte,str,byte_str,raw_str,raw_byte_str}` methods into one method `unescape_literal` with a mode argument.

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Currently, we deal with escape sequences twice: once when we lex a
string, and a second time when we unescape literals. This PR aims to
remove this duplication, by introducing a new `unescape` mode as a
single source of truth for character escaping rules