exerciseFn
Usage: excerciseFn(sym)
Exercises the fn named by sym (a symbol) by applying it to n (default 10) generated samples of its args spec. When fspec is supplied its arg spec is used, and symOrF can be a fn. Returns a sequence of tuples of [args, ret].
Example:
const s = require('speculaas');
const {isBoolean, isString} = s.utils;