Previously we had many different code paths all
handling digests in different places, all with
wildly different digest support. This commit
rewrites this to use a single digest handling
mechanism for all digest operations.
It adds various aliases for digest algorithms,
like node does. For example
`sha1WithRSAEncryption` is an alias for `sha1`.
It also adds support for `md5-sha1` digests in
various places.
