Codes index (#12)

* initial index-support

* make index_handle public only within crate

* re-export CodesIndex in lib.rs for consistency

* make codes_index a feature

* mark featured modules in docs

* reolve cargo doc warnings

* update gh actions

* fix testing in gh actions

* (deosn't compile) extract index binds to separate file

* WIP mostly finish CodesIndex API and conversion to CH

* WIP codes_handle iterator with index works

* compiles but does not allow for editing index
so misses the whole point of index

* actually, the API makes sense (and tests now pass)

* move high-level index tests to integration tests

* add integration tests with multithreading

* add mutex for problematic functions

* add error and panic tests for codes_index

* add features to gh actions

* add index handle interference test

* clippy

* make ec_index default feature

* safeguard all index functions

* make index feature experimental

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Co-authored-by: Robin <saygram@gmail.com>

Author: Quba1