Use a different token other than `'` for referencing rust values

[ColonelThirtyTwo]

Currently the crate uses ' to denote rust variables, which conflicts with single quoted strings. To be more compatible, the crate should use a token that Python does not use to denote rust variables, such as !.

[m-ou-se]

Not supporting single quoted strings has nothing to do with using 'var as placeholders. The crate doesn't support single quoted strings of multiple characters, simply because the Rust tokenizer considers 'a' a character literal, and will therefore reject 'abc'. 'a' works fine, and is parsed very differently from 'a.