Basic statistical computations on frequency maps. A frequency map
(freq-map) is a map from observed values to their frequency in a
data set.

If the observed values are all integers within a small range, then a
frequency map may be exact, such as that returned by
clojure.core/frequencies. Floating-point values or a large range of
integers can be grouped into 'buckets' as in a histogram: the
'bucket-frequencies' function does this.

You can create your own bucketed frequency map (for example, as part
of a larger 'reduce' operation) using the functions 'bucket' and
'recover-bucket-keys'.