slices: handle aliasing cases in Insert/Replace

slices: handle aliasing cases in Insert/Replace

Handle cases where the inserted slice is actually part of the slice
that is being inserted into.

Requires a bit more work, but no more allocations. (Compare to #494536.)

Not entirely sure this is worth the complication.

Fixes #60138

Change-Id: Ia72c872b04309b99025e6ca5a4a326ebed2abb69
Reviewed-on: https://go-review.googlesource.com/c/go/+/494817
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>