internal/abi: common up Method, Imethod, UncommonType types

internal/abi: common up Method, Imethod, UncommonType types

was two commits, the first contained a lot of intermediate work,
better this way.

Change-Id: I7c5b79ef78b21a85828c8aaf9baeae86bb144db7
Reviewed-on: https://go-review.googlesource.com/c/go/+/463118
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>