Use Type Aliases for DoubleWidth Digests

[crichez]

Overview

Digests 128 bits wide and larger should have type aliases to erase the DoubleWidth nesting.

Proposal

public typealias Digest128 = DoubleWidth<UInt64>
public typealias Digest256 = DoubleWidth<Digest128>
public typealias Digest512 = DoubleWidth<Digest256>
public typealias Digest1024 = DoubleWidth<Digest512>

Alternatives

This was originally omitted to make the DoubleWidth nesting as obvious as possible, since it is not a recommended practice and significantly degrades performance. However the same goal can be achieved through better documentation of each type alias without affecting code clarity.