refactor(engine/fstar-ast): get rid of zarith and GMP

[W95Psp]

This helps for #696: this PR gets rid of the dependency to the native lib gmp.
That's virtually the only native lib used by the engine, so we can (almost) consider binaries to be static from now on, that will do the trick at least for MacOS

Technically, this PR:

• remove a bunch of useless code from the F* parser & AST
• kills the dependency to zarith, replacing zarith's integers with... strings. That is fine since those integers are mainly used to represent constants in the AST

[franziskuskiefer]

Nice! Deleting code!