Implementing JWE for P256

[bshambaugh]

I am starting to work through #225 and the first file that I changed was ECDH.ts. Please forgive my deletion of yarn.lock. If this is an issue, perhaps I can address it later. This does not have tests yet, but neither does the prexisting x25519 function. I do need to make sure that when I get a compressed P256 public key as input to getSharedSecret I get the correct results.