* feat(ctb): Refactor LibPosition.sol to rep gindex

Generalized Indices are a more succinct and easy to understand model for
positions in the dispute game tree. Implementation is often more
efficient too and can avoid branching logic for tree traversal.

* update .gas-snapshot

* update .gas-snapshot

* Fix `FaultDisputeGame` tests

* lint; gas snapshot

* Update packages/contracts-bedrock/contracts/dispute/lib/LibPosition.sol
Co-authored-by: clabby <ben@clab.by>

* opt rightIndex

* opt rightIndex

* opt depth

* update gas-snapshot

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Co-authored-by: clabby <ben@clab.by>