This makes it possible to register hasher factories that take a size
hint. Such hashers should produce at least the requested number of
bytes, but may:

1. Produce fewer bytes in some cases (e.g., not enough input is available).
2. May produce more bytes (the usual case).

The caller should check the size of the produced digest to determine if
it's sufficient for the caller's use-case (likely truncating it if it's
too large).

NOTE: the convenience `Sum` function will handle size verification and
truncation internally.