Refactor to use extended numerals.

[ordinarymath]

Cakeml currently uses num option in various places where an extended numeral is more appropriate.

for example in wordlang stack_size is a num option where NONE means the stack is unbounded.
This result in slightly unintuitive defintions written in ways like
OPTION_MAP2 $+ a b
option_le defined with option_le SOME _ NONE.
Proof are also harder where many just end up case splitting on the option.

On brief search in the HOL repo there appears to be
xnum developed in examples/HolCheck/ctlScript.sml.
That should probably be refactored out into a separate file + more syntax sugar to allow stuff like 0e to be a 0 : xnum

[ordinarymath]

Note some more work on enat in HOL4 at examples/computability/kolmog/extNatScript.sml.