Forward referencing in MUF (SF Issue)

[tanabi]

Forward referencing for function names in MUF would be
useful... (To avoid having to stuff the address of
functions into variables to call them out of order)

Also making the entry point of a program the function
named 'main' would allow for organising code in a
cleaner way... (To avoid breaking old code this could be
done via a new preprocessor directive "$entrypoint
<function>", if the directive is missing, the last function
in the program is assumed...)

...

man $entrypoint
$ENTRYPOINT

Sets the function used to start the program. Defaults to the last one.

[aidanPB]

We could borrow DEFER and related words from the Forth-2012 Core Extensions word set.