Add times parameter to digest function

Whilst working on another project involving digest, I came across a need
to hash a value multiple times. Simply rehashing the output of digest
was insufficient as I really needed to rehash the original byte-array.

I added an optional parameter to the digest function to indicate the
number of times the input should be fully ran through the hash function.
This should not break any existing code using this library, as that
parameter defaults to 1. The digest function still returns a properly
padded string signature of the digest.

A couple of tests are included that minimally exercise this expanded
functionality.
1.5.0