This is about equally readable, a lot more terse, and stops
special-casing functions and methods.
```console
$ du -hs doc-old/ doc-new/
671M doc-old/
670M doc-new/
```
This commit makes the `clean::Type::is_same` non-commutative, so
that a generic `impl` matches a concrete return, but a generic return
does not match a concrete `impl`. It makes slice and vector Write
for `u8` not match on every generic return value.
