syntax::token module modernization

[breaking-change]

- Token variant identifiers have been converted to PascalCase for consistency with Rust coding standards
- Some free-functions in `syntax::token` have been converted to methods on `syntax::token::Token`:
    - `can_begin_expr         -> Token::can_begin_expr`
    - `close_delimiter_for    -> Token::get_close_delimiter`
    - `is_lit                 -> Token::is_lit`
    - `is_ident               -> Token::is_ident`
    - `is_path                -> Token::is_path`
    - `is_plain_ident         -> Token::is_plain_ident`
    - `is_lifetime            -> Token::is_lifetime`
    - `is_mutability          -> Token::is_mutability`
    - `to_binop               -> Token::to_binop`
    - `is_keyword             -> Token::is_keyword`
    - `is_any_keyword         -> Token:is_any_keyword`
    - `is_strict_keyword`     -> Token::is_strict_keyword`
    - `is_reserved_keyword    -> Token::is_reserved_keyword`
    - `mtwt_token_eq          -> Token::mtwt_eq`
- `token::Ident` now takes an enum for clarity