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/math/gmp_desc.c

@@ -74,7 +74,7 @@ static unsigned long get_int(void *a)
    return mpz_get_ui(a);
 }
 
-static unsigned long get_digit(void *a, int n)
+static ltc_mp_digit get_digit(void *a, int n)
 {
    LTC_ARGCHK(a != NULL);
    return mpz_getlimbn(a, n);

src/math/ltm_desc.c

@@ -100,7 +100,7 @@ static unsigned long get_int(void *a)
    return mp_get_int(a);
 }
 
-static unsigned long get_digit(void *a, int n)
+static ltc_mp_digit get_digit(void *a, int n)
 {
    mp_int *A;
    LTC_ARGCHK(a != NULL);

src/math/tfm_desc.c

@@ -99,7 +99,7 @@ static unsigned long get_int(void *a)
    return A->used > 0 ? A->dp[0] : 0;
 }
 
-static unsigned long get_digit(void *a, int n)
+static ltc_mp_digit get_digit(void *a, int n)
 {
    fp_int *A;
    LTC_ARGCHK(a != NULL);