This implements a LineReader for line tables that parallels the
existing Reader for debug entries.

This code is partly based on the debug subrepo's fork of dwarf, but it
is a more complete (and, I believe, correct) implementation of the
spec and exposes a more general API.  While the debug subrepo's
implementation exposed only a PC-to-line function, this version
exposes the line table rows to the caller.  This way the caller can
make its own trade-offs when implementing PC-to-line (or line-to-PC),
such as whether or not to build an index for fast lookup.

Change-Id: Ie157bc817f55e940b6f2e1ae010c5a4e1f29c5c8
Reviewed-on: https://go-review.googlesource.com/6734Reviewed-by: Nigel Tao <nigeltao@golang.org>