ANN: 'roll' and '-roll' implementation

Hi PicForthers,

I've implemented the ANS CORE EXT standard word 'roll' - which I'm sure
many have wanted (especially since PicForth has no 'rot' or '-rot'.

Also, I've implemented its non-standard converse, '-roll' (inspired by
FICL Forth, http://ficl.sf.net).

Details and module source file on wiki:
http://wiki.enst.fr/bin/view/Picforth/Roll

IMHO, 'roll' is a very important word for Forth in general, since it
opens up the whole data stack for full read/write random access, picking
up where 'pick' leaves off.

It's relatively expensive for execution time - 24+8n instruction cycles
(including interrupt suspend/restore, where n is the passed argument).

Samuel - please feel free to absorb this into the main PicForth tree
(with or without your optimisations), or leave it on the wiki, or just
ignore, as you wish. If you do absorb it into the main tree, you might
like to introduce an 'autoload' mechanism in the style of what you've
already done with libstore.fs, libfetch.fs etc. All is up to you, since
PicForth is your turf.

-- 
Cheers
David