Start the 'owncrypto' WebCrypto provider.

Implement the generation of RSA signing keys and using them
to sign data with the classic OpenPGP.js implementation of
the crypto routines.
Move the generic WebCrypto funcs where they belong.
Start the key.opgp property hierarchy.
owncrypto: honor modulusLength and publicExponent.

Actually generate RSA keys with the specified parameters.
Store OpenPGP.js WebCrypto data in key.opgp.
Generate (and throw away) an encryption keypair.

This is just a demonstration of how the encryption subkey's
generation will be performed and that does succeed using
either the "owncrypto" or "nfwebcrypto" WebCrypto provider.

Actual inclusion of the encryption subkey into the private and
public key material exported by openpgp.generate_key_pair() is
currently subject to further discussion.
Add a WebCrypto key generation and signing test.