Simplify the macro for forwarding Decoder methods

The old name `implement_ty_decoder!` historically made some sense, but nowadays
is quite misleading for a macro that only forwards Decoder methods.

Requiring the caller to provide the `impl` block makes the calling code
clearer, while also reducing the complexity of dealing with generics. The
downside is a little less context at the macro-definition site, which seems
fine.

The closure-like syntax allows for arbitrary expressions involving `self`,
without running into hygiene problems and without requiring separate closures
for `&self` and `&mut self` methods. It also works well with IDE hover hints.