Allow expression in index and slice expression

It is very useful to allow data to refer to itself.
I propose `bracket-specifier = "[" (number / "*") "]" / "[]"` be extended to `bracket-specifier = "[" (expression / "*" ) "]" / "[]"` for indexes.

`slice-expression = [number] ":" [number] [ ":" [number] ]` be extended to `slice-expression = [expression] ":" [expression] [ ":" [expression] ]` for slices

The scope of the expression is the parent of the object being sliced, this can be extended with `let()`