refect: rename Ptr Kind to Pointer (but keep Ptr)

refect: rename Ptr Kind to Pointer (but keep Ptr)

reflect.Ptr didn't match reflect.UnsafePointer or unsafe.Pointer
so rename it to reflect.Pointer. Keep reflect.Ptr for compatibility.

Likewise with PtrTo.

Change to use it in std will come in a subsequent CL.

Fixes #47651

Change-Id: I5d4abe2b2fe10948bd68bb12c557165bedffbcba
Reviewed-on: https://go-review.googlesource.com/c/go/+/341333
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Joe Tsai <joetsai@digital-static.net>
Reviewed-by: Robert Griesemer <gri@golang.org>