Addition of rudimentary error tracing

[czurnieden]

This adds a compile-time-optional macro that inserts a line to print out the filename, line-number, and function to stderr before all of the goto LTM_ERR; instances and all return err; if err != MP_OKAY.

[czurnieden]

Eases the work quite a bit if the error comes from deep inside a recursive function.
Found it very useful while debugging the fast-radix-conversion at the upper limit of LTM and had to add higher Toom-Cook functions (4 - 7) to avoid waiting a week to get the result. (One MP_OVF had the reason that mp_add adds an extra limb unconditionally, so the maximum number is 2^((MP_MAX_DIGIT_COUNT - 2)*MP_DIGIT_BIT))

Ah, the first typo has already been found :-(