remove dependence on scala-js-dom facade from asymmetric keys

[bblfish]

The branch on which PR #48 is located, used the browser dom web crypto API to get going (in order to make the task more manageable). Node JS uses the same Web Crypto API but somewhat differently, and so it seems better to build a local facade as is done for the symmetric keys.

Implementing #52 will likely depend on this.

Implementing more cryptographic algorithms, such as in #51, would help lay the groundwork of tests to help work out what parts of the facade need to be written.

[armanbilge]

Node JS uses the same Web Crypto API but somewhat differently

Minor clarification: Node.js has its own crypto APIs. Also, in recent versions they've experimentally provided the Web Crypto API as well, but so far there are no stability guarantees for it.

https://nodejs.org/api/webcrypto.html#web-crypto-api

[yilinwei]

This is now stable:2.