Files in the top-level directory from the latest check-in

This is Alexander Ferrieux' implementation of Tcl TIP 424, extracted from the
Tcl sources, along with the necessary internal functions, to provide it as a
separate package.

The sources come from the Tcl 8.6.4 release, but the functions in question
haven't been changed in some time, so this package should work with a number of
previous Tcl versions as well.