Break out the interpreters (simple and forth) w/o ifdefs. This is
akin to what Pedro Souza and Wojciech Koszek did in the lua GSoC with
interp.h, interp_simple.c and changes to interp.c and interp_forth.c,
but completely redone from scratch.

This effectively restores the spirit of r326712 (my first attempt to
bring in Pedro's and Wojciech's work) updated for new requirements
that had silently broke their original work. This change also differs
by using fixed function names instead of function pointers to simply
things. Only one interpreter at a time may be compiled in.

Also of note: we take a mutable string, pass it in via a const char *
pointer into intrp_forth's interp_run(). We then cast away the const
to pass into ficlExec since ficl would require extensive changes to
properly const-poison. See Sections 6.5.2.5 and 6.7.3 of C11 standard
noting it's only UB if you modify a const object through a non-const
pointer, but not char [] -> const char * -> char * as here.