[V2] index argument in methods with suffix Indexed

[dorzepowski]

Current state:
Many methods are accepting iteratee as a function with 2 arguments func(item T, index int)

It's rather often that the index is not needed so the method that could be passed to Map, Filter, Reduce often ends up with with _ int as a second argument, just to match the function struct.

Proposition:

Rename all the current functions which accepts the index to XyzIndexed
Add functions without suffix Indexed which accepts similar function but without index argument.