math: change get_digit() return value

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

src/headers/tomcrypt_math.h

@@ -80,7 +80,7 @@ typedef struct {
      @param n  The number of the digit to fetch
      @return  The bits_per_digit  sized n'th digit of a
    */
-   unsigned long (*get_digit)(void *a, int n);
+   ltc_mp_digit (*get_digit)(void *a, int n);
 
    /** Get the number of digits that represent the number
      @param a   The number to count