[eacsl] Extract some translation functions

- `tapp_to_exp` is moved to `Logic_functions`, with the existing
`Logic_function.tapp_to_exp` renamed to `function_to_exp` and moved as
an internal function;
- `name_of_mpz_arith_bop` is moved to `Gmp`;
- `add_cast` and `strnum` are moved to a new `Typed_number` module.