Add unicode escapes, allow non-ASCII identifiers & others improvements

[nibags]

Changes:

• Fix escape characters. The detection of escapes is very general, I have replaced it with the specific escapes supported by Rust [1].
• Add unicode escapes: \u{NNNNNN} [2]. However, the bytes and byte strings don't allow unicode escapes [3], therefore, the unicode escape is placed in a different object and new objects are created for byte strings and byte characters.
• Fix: remove obsolete integer suffixes is & us (these were depreciated by isize & usize [4]).
• Allow non-ASCII characters in identifiers (in the Rust parser, an identifier is: [a-zA-Z\x80-\xff_][a-zA-Z0-9\x80-\xff_]* [5]). This affects Lifetimes, Macros, functions and types (to do this, I basically added \\x80-\\xFF, and it works fine. But the POSIX character class [:ascii:] can also be used, for example: (?:[a-zA-Z0-9_]|[^[:ascii:]]) ).
• Add missing sigils [6]: ~ @

NOTE: The use of non-ASCII characters in identifiers is a feature of Rust, however these have some issues and aren't currently fully supported (there is even the possibility that it will be eliminated in the future) (see ref. [7]).

If you want to make some change, correction, or undo something, I have no problem.

References:

• [1] Escapes supported in Rust (Rust Docs): https://doc.rust-lang.org/std/primitive.char.html#method.escape_default
• [2] Unicode escapes (Rust RFCs): https://github.com/nox/rust-rfcs/blob/master/text/0446-es6-unicode-escapes.md
• [3] Do not accept unicode escape characters in byte strings or as byte: https://github.com/rust-lang/rust/pull/23625
• [4] Change suffixes to isize and usize: https://github.com/rust-lang/rust/issues/22496
• [5] Identifiers in Rust parser (see line 42): https://github.com/rust-lang/rust/blob/master/src/grammar/lexer.l
• [6] Sigil reference: https://github.com/rust-lang/rust-wiki-backup/blob/master/Sigil-reference.md
• [7] Issue for non-ASCII identifiers: https://github.com/rust-lang/rust/issues/28979

Some Sources:

• Rust in Vim: https://github.com/rust-lang/rust.vim/blob/master/syntax/rust.vim
• Rust in ACE: https://github.com/ajaxorg/ace/blob/master/lib/ace/mode/rust_highlight_rules.js

[zargony]

Thanks for submitting this changes! And sorry for the late response...

Unicode escapes look good to me. It would be nice to also have it covered by tests.

Are ~ and @ sigils are still used? I though they were deprecated a long time ago?

[nibags]

Yes, @ and ~ are deprecated! I think I better remove them. I also added the tests

[nibags]

Sorry, the test I did was making mistakes and I made unnecessary commits