[chain] keychain module doesn't need to exist

[LLFourn]

The module tries to explain itself as follows:

A keychain here is a set of application-defined indexes for a miniscript descriptor where we can derive script pubkeys at a particular derivation index.

Which is very difficult to read. More importantly it only has one thing in it KeyChainTxOutIndex. We can just have keychain_txout_index.rs. We could move indexers into their own module so indexers/keychain_txout.rs, indexers/spk_txout.rs.

[evanlinjin]

Let's do this after #1203. I'll take responsibility for the convoluted crate docs.

[ValuedMammal]

@evanlinjin If you want I can open a PR and have you make suggestions for the crate docs.. Actually, removing this module means we won't need the old docs. This also gives us a chance to finally relocate the Balance struct