Rust: Add i128 & u128, and allow underscores in unicode escapes

Changes in Rust highlighting:

• Add missing types (keywords) and missing integer suffixes: i128 & u128
  • https://doc.rust-lang.org/std/primitive.i128.html
  • https://doc.rust-lang.org/std/primitive.u128.html
• Fix: Allow underscores in unicode escapes: https://github.com/rust-lang/rust/pull/43716

Sources:

• Rust documentation: https://doc.rust-lang.org/
• 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