Clean-up interface of `COEFF_TO_PTR` and `COEFF_IS_MPZ`?

[user202729]

Is your feature request related to a problem? Please describe.
Currently, COEFF_TO_PTR is supposed to take in a fmpz = slong instead of a fmpz_t = fmpz[1], but if you accidentally pass a fmpz_t into it, it will still compile, but does the wrong thing (usually segmentation fault) later.

COEFF_IS_MPZ has a similar issue, but at least it will compile error instead of segmentation fault.

In addition to the segmentation fault, this leads to code of the form

    fmpz c = *f;

    if (!COEFF_IS_MPZ(c))    /* f is small */

Describe the solution you'd like
I suggest doing one of the following

• Use _Generic macro (requires C11 or later) to make it accept both fmpz and fmpz_t.
• make them a FLINT_FORCE_INLINE function to allow type checking.