BFI: Clean up BlockMass

Implementation is small now -- the interesting logic was moved to
`BranchProbability` a while ago.  Move it into `bfi_detail` and get rid
of the related TODOs.

I was originally planning to define it within `BlockFrequencyInfoImpl`
(or `BFIIBase`), but it seems cleaner in a namespace.  Besides,
`isPodLike` needs to be specialized before `BlockMass` can be used in
some of the other data structures, and there isn't a clear way to do
that.