Counts values which satisfy a test.
This is part of package extra-array.
array.count(x, ft);
// x: an array
// ft: test function (v, i, x)
const array = require("extra-array");
var x = [1, 1, 2, 2, 4];
array.count(x, v => v % 2 === 1);
// 2
array.count(x, v => v % 2 === 0);
// 3