reflect: allow conversions between slices of named {byte,rune} and string

reflect: allow conversions between slices of named {byte,rune} and string

So the reflect behavior matches that of the language.

These conversions are allowed:

[]myByte <-> string
[]myRune <-> string
[]myByte <-> myString
[]myRune <-> myString

And even if the left-hand-side is named, e.g.

myBytes([]myByte) <-> string

Fixes #53523

Change-Id: I6562e72bc233a45dc7b02f75f68020831ad399ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/739680
Auto-Submit: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>