internal/memoize: show key type in panics
internal/memoize: show key type in panics

gopls uses a lot of keys that are just strings, which print without any
type information. Include the type of the key explicitly.

Updates golang/go#41415.

Change-Id: I01cfc685184e7b44c1f562b6536f173da5ae4830
Reviewed-on: https://go-review.googlesource.com/c/tools/+/255357
Trust: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools