Add crypto.key module

[lukeed]

The current worktop/crypto module has keyload and keygen methods, but they're really not all that helpful. I end up dropping into crypto.subtle.importKey (and others) directly quite often, which is a good signal that something should be done here.

Additionally, the native TS definitions for importKey et all are pretty useless... With custom methods, I should be able to accurately offer strict overloads so that you can only have/define valid combinations. For example, for importKey has its own restrictions and generateKey has different input requirements based on format. Both of these can offer much better types - and possibly a tiny abstraction.