FIX: flatten args passed to Callable[…].copy_with

[j-ittner]

This PR fixes an edge case for constructing copies of parameterised instances of generic type Callable:
Callable.copy_with() expects a flat tuple (arg_1, arg_2, ..., arg_n, return) instead of ([arg_1, arg_2, ..., arg_n], return), so we need to flatten the list of arguments prior to calling copy_with.