This change adds code, originally written by Russ Cox <rsc@golang.org>
and open-sourced by Google, that converts from the "legacy"
binary pprof profile format to a struct representation of the
new protocol buffer pprof profile format.

This code reads the entire binary format for conversion to the
protobuf format. In a future change, we will update the code
to incrementally read and convert segments of the binary format,
so that the entire profile does not need to be stored in memory.

This change also contains contributions by Daria Kolistratova
<daria.kolistratova@intel.com> from the rolled-back change
golang.org/cl/30556 adapting the code to be used by the package
runtime/pprof.

This code also appeared in the change golang.org/cl/32257, which was based
on Daria Kolistratova's change, but was also rolled back.

Updates #16093

Change-Id: I5c768b1134bc15408d80a3ccc7ed867db9a1c63d
Reviewed-on: https://go-review.googlesource.com/32811
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>