rust-index: synchronize remove to Rust index Future steps will bring the two indexes further together until we can rip the C index entirely when running Rust code.

rust-index: remove `__setitem__` method from the mixed index This is not defined on the Python or C one, and isn't used anywhere.

rust-index: check equality between rust and cindex for `__len__`

rust-index: synchronize append method We now append to the Rust index just as we do to the C index. Future steps will bring the two indexes further together until we can rip the C index entirely when running Rust code.