This uses#481 (and is actually why I made#481).

I still need to write the docstring.

I tried to make this efficient by performing repeated squaring. For example, forA.power(8), this will compute:

cur=op(A @A).new()cur<<op(cur @cur)updater<<op(cur @cur)

This makes the implementation a little... uh... not obvious, but it works well.

A.power(n) returns anexpression so mask can be applied. Even if a mask is used, an intermediate resultA.power(n // 2) can still be pretty large--we only apply the mask on the very last matrix multiply.

I decided to requiren to be a positive integer.n == 0 could return "the" identity, but what would be the identity for different semirings?

This method will let us easily and efficiently implementnumber_of_walks ingraphblas_algorithms.