garbage collector & hoisting

[somucheffort]

1. garbage collector
   currently, coco has some type of scoping with hashmap stored variables (which is probably great for toy interpreter), but it'll be better if there was a garbage collector.

2. hoisting
   js has function hoisting.

// sort of arrow functions will be introduced later, because those `let sum = (a, b) => a + b` is horrible and cannot be hoisted.
let preHoist = hoistedSum(1, 2)

fun hoistedSum(a, b) -> a + b

let postHoist = hoistedSum(3, 4)

log(preHoist, postHoist) // 3 7