Tx_rollup,Proto: simplify the indexes type

Context

I honestly don't remember the reason why I chose sets to handle the indexes. If the index belongs to the context, we know that it is Created. We can simply add to the list when Created and ignored when it is Existed. I also added several tests about the indexes.

Note that this not really critical, an RPC could ask the context which index is allocated for a given address, and thus, get the associated proof.

Manually testing the MR

$ cd src/proto_alpha/lib_protocol
$ dune exec test/unit/main.exe -- test "tx rollup l2 apply" --verbose | grep -v "SKIP"
$ dune exec test/pbt/test_tx_rollup_l2_encoding.exe