This includes methods for mean, quantile, median, var, std, cov and cor,
plus new functions skewness and kurtosis, and weight types.
Code is copied from StatsBase with some cleanup where needed, in particular
for dispatch, to move from `@nloops`/`@nrefs` to cartesian indexing and to
be closer to the mapreducedim code.
Weights are now passed via a keyword argument rather than by dispatching on
AbstractWeights, so as to support any array where all weights types
give the same result.

…rable