Cleaning up the network constants

[clarkmoody]

The Network enum contains the human-readable constants for a few chains, and there are more that are not included yet.

Since Network is a public enum, and new coins would be breaking changes, requiring a version bump.

There are a couple ways forward:

1. Break out Network into a new crate / repo, and link against an early version, not caring about newer coins.
2. Remove the concept entirely, leaving the hrp as a string input for the calling code.
3. Keep Network, but throw away all the coins except Bitcoin and Testnet. It's tough luck for other chains after this.

Thoughts?

[clarkmoody]

Note: I created a placeholder repo to go down the 1. route above.

[sgeisler]

What do you think about rust-bitcoin/bitcoin-constants#1?

[clarkmoody]

@sgeisler It looks good so far. I guess we can add more fields as we go along: address prefixes, private key version bytes, etc.