Fixed a few limit cases in some distributions (e.g. (uniform-dist 0 0) didn't
act like a delta distribution, (beta-dist 0 0) and (beta-dist +inf.0 +inf.0)
pretended to be defined by unique limits even though they can't be)
Made integer distributions' pdfs return +nan.0 when given non-integers
Added "private/statistics/counting.rkt", for hashing and binning samples
Added `flvector-sums' (cumulative sums with single rounding error)
Added `flinteger?', `flnan?' and `flrational?', which are faster than their
non-flonum counterparts (at least in Typed Racket; haven't tested untyped)
2d34811ab6