[RF] Vectorize `RooAbsBinning` interface for bin index lookups

The `RooAbsBinning` interface for bin index lookups was changed to
enable vectorized implementations. Instead of having the override
`RooAbsBinning::binNumber()`, the binning implementations now have to
override the `RooAbsBinning::binNumbers()` function to evaluate the bin
indices of multiple values in one function call.

The interface of `RooAbsBinning::binNumbers()` is designed to facilitate
the accregation of bin indices over multiple dimensions.

The former `RooAbsBinning::binNumber()` method is now implement in terms
of the vectorized version, such that we automatically get wide test
coverage. It was verified that this doesn't come with a performance
overhead.

This will greatly facilitate the vectorization of the RooHistPdf and
RooHistFunc later.