x/debug/core: add initial implementation

Add library for reading core dumps.

Update golang/go#21356

Change-Id: Ia7fa09d9e78a7c30fbdec184a5cf6f73b0994c4a
Reviewed-on: https://go-review.googlesource.com/72830
Reviewed-by: Peter Weinberger <pjw@google.com>
9 files changed