regexp: limit the capacity of slices of bytes returned by FindX

This change limits the capacity of the slices of bytes returned by:

- Find
- FindAll
- FindAllSubmatch

to be the same as their length.

Fixes #30169

Change-Id: I07b632757d2bfeab42fce0d42364e2a16c597360
Reviewed-on: https://go-review.googlesource.com/c/161877


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>