Major refactoring of InterestRateIndex types

- CashIndex & IBOR parameterised by `currency` field
- AONIA, EONIA, etc alias CashIndex{AUD}, CashIndex{EUR} etc. Same for
AUDBBSW re IBOR{AUD}.
- Can now create EURLIBOR/EURIBOR and JPYLIBOR/JPYTIBOR with `libor`
flag in constructor. These are also aliased.