XRun (added in db941f12) and Runx (added in 486ede30) run a function and
convert error <-> exception back and forth. However sometimes it is
handy to only convert a function but not run it - e.g. for passing into

	x/sync/errgroup.Group.Go

To do so this patch adds XFunc and Funcx - functional counterparts to
XRun and Runx.

No new tests are needed because now XRun and Runx are just tiny wrappers
around new functions and we already have tests for XRun and Runx.