Cosmos address (Bech32) support

[tarcieri]

Right now all Ed25519 public keys are represented as colon-delimited hex strings:

https://github.com/tendermint/kms/blob/master/src/ed25519/public_key.rs#L28

They need to be switched over to Bech32 before launch, and ideally before GoS if we plan for KMS to be available for that.

Note: I've been working on a constant time Bech32 implementation for subtle-encoding if there's any interest in using that. Constant time isn't needed here, but subtle-encoding does so happen to already be a KMS dependency.

[jackzampolin]

Would there be required changes elsewhere? If we need this pre-GoS then we need to prioritize these changes cc @zmanian

[zmanian]

I'll make a pull request

[zmanian]

Closed by #71