unsigned long is 32bit wide when compiling with the compiler flag "-mx32"
but the digit size of the math libraries is still 64 bit which lead to
the buggy ecc code.
Therefore define a new type ltc_mp_digit with the correct width and use
that as return value of get_digit()
Has been tested with all three math providers
addmod and submod are moved to the end of the math descriptor, in order
to be able to run existing software against a new version of ltc without need
to rebuild the software.
