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.