PE specification requires that long section and symbol names
are stored in PE string table. Introduce StringTable that
implements this functionality. Only string table reading is
implemented.

Updates #15345

Change-Id: Ib9638617f2ab1881ad707111d96fc68b0e47340e
Reviewed-on: https://go-review.googlesource.com/22181
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>