Codegen token-related functions and provide quote! as a lightweight proc-macro

[udoprog]

The new quote! macro doesn't make use of quote or syn parsing internally, and uses codegened lookup tables for keywords and punctuations. It's also not implemented as a macro_rules! as previously.

This accomplishes the following things:

• The proc macro is as fast and uses as little resources as it can.
• $ is a reserved punctuation in macro_rules!, so converting quote! into a proc macro allows us to use it.
• quote! produces an instance of Quote<'a> that implements ToTokens, which is a stored environment that can perform the specified quoting. This avoids copying a token stream excessively when nesting quoting.