The same is done for `Vector` (and thus `Packed*Array`).
`begin` and `end` can now take any value and will be clamped to
`[-size(), size()]`. Negative values are a shorthand for indexing the array
from the last element upward.
`end` is given a default `INT_MAX` value (which will be clamped to `size()`)
so that the `end` parameter can be omitted to go from `begin` to the max size
of the array.
This makes `slice` works similarly to numpy's and JavaScript's.
