Refactor to use extended numerals.

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