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`

Start by reading examples/HolCheck/ctlScript.sml, where xnum is already developed, and inspect CakeML uses of num option such as wordlang stack_size. The issue suggests extracting xnum into a separate file and adding syntax such as 0e, but the required scope and completion criteria need to be settled first.

Written by the indexing model from the issue text.