Receiver parameters generally aren't relevant to the function
signature type. In particular:

  1. When checking whether a type's method implements an interface's
     method, we specifically want to ignore the receiver parameters,
     because they'll be different.

  2. When checking interface type equality, interface methods always
     use the same "fakethis" *struct{} type as their receiver.

  3. Finally, method expressions and method values degenerate into
     receiver-less function types.

The only case where we care about receiver types matching is in
addmethod, which is easily handled by adding an extra Eqtype check of
the receiver parameters. Also, added a test for this, since
(surprisingly) there weren't any.

As precedence, go/types.Identical ignores receiver parameters when
comparing go/types.Signature values.

Notably, this allows us to slightly simplify the "implements"
function, which is used for checking whether type/interface t
implements interface iface. Currently, cmd/compile actually works
around Eqtype's receiver parameter checking by creating new throwaway
TFUNC Types without the receiver parameter.

(Worse, the compiler currently only provides APIs to build TFUNC Types
from Nod syntax trees, so building those throwaway types also involves
first building throwaway syntax trees.)

Passes toolstash -cmp.

Change-Id: Ib07289c66feacee284e016bc312e8c5ff674714f
Reviewed-on: https://go-review.googlesource.com/20602Reviewed-by: Robert Griesemer <gri@golang.org>