fix off-by-one error introduced in bd7d7a2061
tailcall now running in a simpler model, with no eval-flags and no nre-stack rewriting; yieldto also requires one fewer bounce. Mostly from mig-nre-mods
Name functions according to 'what' instead of 'how' in the [tailcall]
machinery, in view of making public some parts of it.
Test for Bug 1884496 (not buggy on trunk).
Put back TclBackgroundException in internal stub table, so extensions using this, compiled against 8.5 headers still run in Tcl 8.6.
merge trunk
Remove various double-defined (both in public and private stub tables) functions from private stub table