Ensure rust keywords are escaped

[glihm]

There are some keywords in rust that are allowed in cairo (like type).

The idea of this issue is to find an elegant solution to escape rust keywords when struct or enum members/variants are expanded. But here are some limitations:

1. With proc-macro2, we can't cast the string r#type into an Ident. It's an invalid identifier.
2. Also, when using quote!, we can't do quote!(r##name).

Currently there's a workaround only for the type keyword.

[glihm]

More keyword were merged in #16.