exponent noation real parsing

[joe-hauns]

Fixed the parsing of exponent notation integers that was broken by introducing gmp as @quickbeam123 pointed out.
Now we can parse problems like

tff(p_real_type,type,
    p_real: $real > $o ).

tff(reals,axiom,
    ( p_real(123.456e78)
    | p_real(123.456E78)
    | p_real(-123.456E78)
    | p_real(123.456E-78)
    | p_real(-123.456E-78) ) ).

again.

[joe-hauns]

(this one depends on the signature PR)