Refactor identity-sequencer into world-tree

[0xKitsune]

This PR refactors the identity-sequencer crate to be simpler and more compact. Below is a summary of the significant changes that occurred during the refactor.

• Originally, this repo was imagined to be a modular replacement for the signup-sequencer. Over time, this effort has evolved to focus on modularizing specific parts of the signup-sequencer rather than replacing all functionality. Since this repo does not contain logic for sequencing identities and to avoid confusion, the main crate was renamed to world-tree.
• The sequencer crate was removed since this repo will not contain any logic related to sequencing identities.
• The tree-availability crate and the state-bridge crate were combined into the world-tree lib and now exist under the tree and state_bridge modules respectively.

[Dzejkop]

Very nice!