Re-name `corepc-client`

[tcharding]

The corepc-client name is pure fire but currently its used for an integration test client. Now that we are working on a bare bones prod client (#505) we could consider giving up the name.

1. Should we use corepc-client for the prod client?
2. If yes, what name should we use for the integration test client? One idea corepc-regtest-client.

While we are at it I rekon probably best to split the two clients into separate crates. There is no real benefit in having them together.

[jamillambert]

I like both ideas. I'm not sure of the effect on downstream though, but maybe it's fine. bitcoind and electrsd depend on corepc-client and publicly reexport it so releasing an update pointing to the new test client should be fine for their downstream. And surely they have to be in separate crates, otherwise it is just how it is now (after #505) with both being in corerpc-client.

[tcharding]

I'm not sure of the effect on downstream though

It wildly breaks downstream :) But if it is in integration tests then its not hard for them to fix and if its in production then I don't care because the crate specifically says 'do not use this in production'.