gocore: add gocore reader

Add a library that can extract a bunch of information from a Go core dump.

Update golang/go#21356

Change-Id: I9997c80452a4ad205d0799f89b2e268b4a5a9e2f
Reviewed-on: https://go-review.googlesource.com/73191
Reviewed-by: Austin Clements <austin@google.com>