Apologies for the enormous commit, but I don't see a way to split this
up nicely. In the vast majority of cases it's a simple change. A few
extra places can use TRY instead of manual error checking though. :^)
This is a simple implementation of SubtleCrypto.digest() using LibCrypto
under the hood, so it supports all the required hash functions:
SHA-1, SHA-256, SHA-384, SHA-512.
Two FIXMEs remain: doing the hashing "in parallel", and supporting an
object argument instead of a plain string.
Just some boilerplate code to get started :^)
This adds both the SubtleCrypto constructor to the window object, as
well as the crypto.subtle instance attribute.
Since we don't support IDL typedefs or unions yet, the responsibility
of verifying the type of the argument is temporarily moved from the
generated Wrapper to the implementation.
