Export functions that client would use and adjust their names taking
into account that now there is a package prefix. So e.g.

	raise		-> exc.Raise
	errcatch	-> exc.Catch
	erronunwind	-> exc.Onunwind
	...

It is completely orthogonal to exception handling.

It is

	xruntime.Traceback()

Usage would be:

	myname.Func(),	and
	myname.Pkg()

This is pristine import. Files are taken as-is from git-backup 3ba6cf73.
Will adapt them to live in separate package in the next commit.

[1] https://lab.nexedi.com/kirr/git-backup

This will be a place to keep handy go bits shared across projects.