chore: move Balance out of keychain module

[startup-dreamer]

Description

Tries to Fix: bitcoindevkit/bdk_wallet#128

Moved Balance from keychain.rs to tx_graph.rs and did the rexport for bdk::wallet module.

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[evanlinjin]

Thank you for this contribution.

However, the discussion about where to put everything in bdk_chain is not final. I rather we move everything in one PR (after we come to consensus, which has not happened yet).

For the meantime, I will provide my thoughts on the matter. I think we should have a file chain/types.rs that contains all smaller public traits and structs (replacing chain_data.rs and tx_data_traits.rs files). Balance should be in types.rs.

Also note that this is a duplicate of: #1175

[LLFourn]

It would be great to avoid a types.rs. This is totally unhelpful name because everything we declare is a type. It's fine to put things in lib.rs if there's no where else it belongs.

It would be fine to put tx_data_traits.rs into chain_data.rs. I don't recall why tx_data_traits.rs was created.

[startup-dreamer]

Closing this in support of #1283 from This comment