[Huf_hist] greedily compute ordinal feature histograms

This module uses a greedy online version of the the Huffmann
algorithm to compute the optimal histogram of a numeric feature.
using module Huf_hist to downsample features whose cardinality is above the feature cardinality cap.  Huf_hist is cleverer than the previous approach, with improvement increasing with the skewness of the feature distribution.
documentation and 'stylistic cleansing'
fix: when search for features by name, consider the possibility that several features satisfy the search, as feature names may not be unique
newline
using tailrec List.map_rev
got rid of 'shrink-first-tree' option, as it makes zero sense.