Add fexp, please #211

pforth has the floating point natural log function, FLN, but does not have not FEXP, the floating point exponential function. This is to respectfully request that FEXP be added to pforth.

The rationale for this request is (a) that these functions are, or course, inverses of each other; (b) pforth aspires to ansi forth status or equivalence, and fexp is in the Forth standard.

I would also note another similar omission in pforth: it has FLNP1 but is missing the -- also standard -- corresponding FEXPM1. I would ask that it also be added.

End of request. Thanks very much for pforth!