compiler: initial support for exporting function bodies

Create a framework for putting function bodies in export data.  At
present only empty functions will be put there, and they will be
ignored on import.  Later patches will get this to the point of
supporting inlining of (some) functions defined in other packages.

Change-Id: Iab6d0d59f36b9f909a86e649aa79a4d8c068a9c9
Reviewed-on: https://go-review.googlesource.com/c/150061
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
9 files changed