Add modular method of injecting the needed ecc interfaces

[motorina0]

Related to comment here: bitcoinjs/bitcoinjs-lib#1742 (comment)

Just checking if:

• it is OK to make ecpair modular (following the bip32 lib model)?
• someone else is not already working on this

[junderw]

Yes. Go ahead. I tried a couple times but couldn't really find a way that felt good.

Feel free to mess around with the interface / returning something other than the class itself. (ie. bip32 returns an object with methods to get a BIP32 instance, but not the class itself.)

[motorina0]

PR open.
If the changes in the PR are approved then I would also like to update bitcoinjs-message (for selfish reasons).
Would that be OK?

[junderw]

Sure. modular ecc is fine, but we can't add schnorr to bitcoinjs-message in any way, since there's no standard for message signing.