proc: use quote! instead of string-based formatting.

[Dirbaio]

This is a prerequisite for #70

The $crate token is "special", you can't convert it to a string and
back, you have to keep its identity. Using quote! everywhere will allow this.

[jannic]

I don't get in what sense $crate is special or why there is a difference between code: TokenStream = "$crate".parse().unwrap() and code: TokenStream = quote!($crate). (Which doesn't mean that you are wrong. It only shows that I don't understand rust macros.)

Anyway, the suggested code using quote! is easier to read, so I'd say it is an improvement, even if it's not strictly necessary.

[Dirbaio]

(rebased)

[Dirbaio]

see #72.

$crate in decl macros expands to a magic Ident token that refers to the crate that produced it, no matter where it ends up. This works even if that crate is not a dep of the current crate. That's something you can't do with regular program text, you simply can't name the crate because it's not in the current namespace.

This "which crate produced this token" is some "metadata" that rustc attaches to the token. $crate from crate foo and $crate from crate bar are not the same. They both stringify to "$crate" but this metadata is different. If you try to parse "$crate" from a string it doesn't work because it can't know what crate to put in the metadata.