compiler: initial support for exporting function bodies

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.

    Reviewed-on: https://go-review.googlesource.com/c/150061

From-SVN: r266490