as-sha256: support totalLength parameter

[twoeths]

Is your feature request related to a problem? Please describe.

Some consumers only want first n bytes after calling digest() and they have to use this pattern: digest(something).slice(0, n), note that Uint8Array.slice() returns a copy of the array so it's not efficient

Describe the solution you'd like

Add an optional totalLength parameter to digest() methods so it requires only 1 memory allocation

[wemeetagain]

I don't think this is necessary since Uint8Array.subarray is also available.