* Stub for future 'identity' optimisation
I plan to add this later, but I'm using for now for
NaturalToInteger and IntegerToNatural
* Refactor `%builtin`
fixes#1799
- automatically optimise all Natural shaped things
- NaturalToInteger and IntegerToNatural now use
new `Identity` flag (internal use only for now)
which signals the function is identity at runtime
* Use NaturalToInteger and IntegerToNatural for Nat and Fin
Also define show fin in terms of finToInteger, for speed
* Fix name handling for %builtin
* [ tests ] fixes + #1799
* remove %builtin from libs
Add back after next version
* Use resolved names where convenient
