The recursive-lengths-list-many-files function returns a list
of numbers. Each number records the length of a function definition.
What we need to do now is transform this data into a list of numbers
suitable for generating a graph. The new list will tell how many
functions definitions contain less than 10 words and
symbols, how many contain between 10 and 19 words and symbols, how
many contain between 20 and 29 words and symbols, and so on.
In brief, we need to go through the lengths’ list produced by the
recursive-lengths-list-many-files function and count the number
of defuns within each range of lengths, and produce a list of those
numbers.