Add cast function. (#944)

Allows users to cast between types:
```py
assert client.get(std.str.cast(std.int64(1))) == "1"
assert client.get(std.array[std.str].cast(
    std.array[std.int64](
        [std.int64(1), std.int64(2), std.int64(3)]
    )
) == ["1", "2", "3"]
```

Supports scalars, enums, array, tuple, range. Multiranges not supported, not sure we can even construct them in the qb right now.