cmd/pprof: add Go implementation

Update issue 8798

This is a new implementation of pprof,
written in Go instead of in Perl.
It was written primarily by Raul Silvera and
is in use for profiling programs of all languages
inside Google.

The internal structure is a bit package-heavy,
but it matches the copy used inside Google, and
since it is in an internal directory, we can make
changes to it later if we need to.

The only "new" file here is src/cmd/pprof/pprof.go,
which stitches together the Google pprof and the
Go command libraries for object file access.

I am explicitly NOT interested in style or review
comments on the rest of the files
(that is, src/cmd/pprof/internal/...).
Those are intended to stay as close to the Google
copies as possible, like we did with the pprof Perl script.

Still to do:

- Basic tests.
- Real command documentation.
- Hook up disassemblers.

LGTM=r
R=r, bradfitz, alex.brainman, dave
CC=golang-codereviews
https://codereview.appspot.com/153750043