Symconst

WARNING: Latest commit would probably break other dependencies.

This change is intended for Nano-js:

- Before: SymConsts were dumped directly to the fp file, through `toFixpoint` (Language/Fixpoint/Types.hs), instead of being added with the rest of the literals.

- NanoJs needs symbolic literals to be added along with the rest of the literals, so that all sorts are unified to FInt before moving to Fixpoint.