Added add and multiply inline operations

[bkrogh]

I have a case, which I consider fairly common:
Multiply or add a number smaller than WORD_SIZE bytes, to a given bignum (myBigNum in below example).

The library currently supports this by doing something like this:
int k = 1234;
struct bn tmp;
bignum_from_int(&tmp, k);

struct bn intermediate;
bignum_mul(&tmp, &myBigNum, &intermediate);
bignum_assign(&myBigNum, &intermediate);

The multiply operation is quite expensive because of internal O(n^3) implementation. When doing a large number of these operations it will dominate the execution time.

My proposal here is to add two simple inline operations that multiply with a DTYPE integer.
These are much faster than regular add/mul, and a bit more convenient in usage. The above code becomes:
bignum_mul_int(&myBigNum, k);