Dedupe source files

[hurrymaplelad]

Depends on #137

This diff tackles point 2 from #137 - lots of crypto code is duplicated.

This problem seems worth solving both to make sure the crypto functions evolve consistently, and to reduce merge and rebase pain for contributors.

See transcludeModule for the crux of this diff. We use a very simple CommonJS -> browser transform to share some pure functions between the node CLI and browser.

The extra separation between codec.js and impl-cryptojs.js paves the way for easily switching between CryptoJS and WebCrypto.

[robinmoisson]

This is looking really interesting - I'll need more time to look into it in detail. Again, thank you for the great PRs and comments!

[hurrymaplelad]

FYI @robinmoisson - rebased onto the merged version of #137. Should be much less noisy to review now!

[robinmoisson]

Hey @hurrymaplelad, thanks again for your PR and proposals! Nice job managing to refactor common code between node and browser runtimes.

I think this will allow to support multiple implementations and opens the door to new features like #112 (where suddenly the same decryption logic can easily by used in CLI or in browser).

I had some comments about moving things that were hard to convey, so I created a second proposal here #141 instead. Feel free to comment there if you have any thoughts. Thank you!