FixedArray::deep_clone is NOT really a deep clone

[CAIMEOX]

The implementation of deep_clone in fixedarray.mbt is not really a deep clone because it won't clone elements.

core/array/fixedarray.mbt

Line 958 in e687864

fn deep_clone[T](arr : FixedArray[T]) -> FixedArray[T] {

Unexpected behavior happens when there is internal mutability:

test {
  let arr : FixedArray[_] = [@ref.new(1), @ref.new(2), @ref.new(3)]
  let cloned = deep_clone(arr)
  arr[0].val = 0
  inspect!(cloned, content="[{val: 0}, {val: 2}, {val: 3}]")
}

[illusory0x0]

@CAIMEOX

• add Clone trait #1429