Allow 128 bit indices

[rumschuettel]

https://github.com/ZKSI/QI.jl/blob/53c59ff12e49b69558529275bdd0e974115fae74/attic/src/sparse/base_sparse.jl#L1

Hi there,

great work! Could you by any chance allow the use of a custom index type? This makes a lot of sense e.g. for extremely shallow circuits, where the state vector is very sparse throughout but could e.g. be on 100 qubits; BigInt would be awesome, but if that's an issue then maybe at least Int128, as an option? And maybe even allow Int32, for relatively dense sparse matrices -- saves a lot of memory.