Add {lists,strings}.At flow ops to access elements of a List and runes in a string

[Catya3]

Removes built-in Index

Needed for https://github.com/Catya3/mandelbrot

[Catya3]

I realized we have lists.Index already

[Catya3]

Uhh we still need this. I don't believe Index works for me as the generic types are not exact.

// Index returns the element at the given index in the ordered collection.
// If the index is out of bounds, it returns an error.
// The index is zero-based.
#extern(index)
pub flow Index<T list<any> | string>(data T, idx int) (res T, err error)

res T is a list not an any or even $element_type