Make the reflect path parser utf-8-unaware

[nicopap]

Objective

All delimiter symbols used by the path parser are ASCII, this means we can entirely ignore UTF8 handling. This may improve performance.

Solution

Instead of storing the path as an &str + the parser offset, and reading the path using &self.path[self.offset..], we store the parser state in a &[u8]. This allows two optimizations:

1. Avoid UTF8 checking on &self.path[self.offset..]
2. Avoid any kind of bound checking, since the length of what is left to read is stored in the &[u8]'s reference metadata, and is assumed valid by the compiler.

This is a major improvement when comparing to the previous parser.

1. access_following and next_token now inline in PathParser::next
2. Benchmarking show a 20% performance increase (Add reflect path parsing benchmark #9364)

Please note that while we ignore UTF-8 handling, utf-8 is still supported. This is because we only handle "at the edges" what happens exactly before and after a recognized SYMBOL. utf-8 is handled transparently beyond that.

[github-actions]

Example alien_cake_addict failed to run, please try running it locally and check the result.

[soqb]

not a huge fan of the unsafe, but it's reasonably trivial and i trust the performance improvements are compelling!

[nicopap]

ooops. Well I rebased to latest HEAD, expecting to fix CI. But instead I disabled auto-merge.