[derive] Simplify code, remove obsolete features

[joshlf]

Clean up the implementation, especially in fn impl_block. Make the following notable changes:

• Previously, syn didn't support parsing macro invocations in const generics without the full feature enabled. To avoid the compile-time overhead of that feature, we worked around it by constructing AST nodes manually. syn has since added support for this without requiring the full feature, so we make use of it.
• We used to need to split types into those that transatively depended upon type generics (like [T; 2]) and those that didn't (like [u8; 2]). We made a change in Move field type bounds into where clauses #119 that made this distinction irrelevant, but we never removed the code to perform the split. In this commit, we remove that code. That code was the only reason we needed to enable syn's visit feature, so we are also able to remove that feature dependency.