regexp: hide one-pass code from exported API

Update #8112

Hide one-pass regexp API.

This means moving the code from regexp/syntax to regexp,
but it avoids being locked into the specific API chosen for
the implementation.

It also removes a slice field from the syntax.Inst, which
should avoid bloating the memory footprint of a non-one-pass
regexp unnecessarily.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews, iant
https://golang.org/cl/98610046