parser: Split `+=` into `+` and `=` where `+` is explicitly requested (such as generics)
Added functions in tokens to check whether a token leads with `+`. Used them when parsing to allow for token splitting of `+=` into `+` and `=`.
Fixes https://github.com/rust-lang/rust/issues/47856
The syntax crate contains those things concerned purely with syntax
– that is, the AST ("abstract syntax tree"), parser, pretty-printer,
lexer, macro expander, and utilities for traversing ASTs.
For more information about how these things work in rustc, see the
rustc guide: