ffi: Follow example of C_call_scheme and eliminate aborts.

ffi: Follow example of C_call_scheme and eliminate aborts.

Punt pop_return_p parameter to Interpret; instead, leave #f in EXP and
Re_Enter_Interpreter, like C_call_scheme.  Keep callouts simple, fast.
Make callbacks save/restore machine state (last_return_code,
C_Frame_Pointer, C_Stack_Pointer, LEXPR_ACTUALS and PRIMITIVE) and use
RC_END_OF_COMPUTATION, also like C_call_scheme.

Add a gc-flip to the test callback.