Add trait defining kotlin-like scope functions.
Currently provided functions are:
transform(): transform value to another.
rust
let v = gen_rand(0, 10).transform(|v| 'x'.repeat(v));
// => `v` is random length of "xx...".
modify(): modify the value and return.
rust
let v = vec![1, 2, 3].modify(|v| v.push(4));
// => `v` is [1, 2, 3, 4].