[HandshakeToFIRRTL] Add option for Index bit width

[mortbopet]

Currently, any index-typed values will be emitted with a fixed width, currently set to 64 bits. Unsurprisingly, testing shows that this can have a huge overhead in circuits. Eventually we want to have a proper (path- and flow sensitive) width-infererence pass to take care of this. Until then, it would be nice to have an option to the HandshakeToFIRRTL pass to specify what width these signals should be emitted with.