coerce fn() to fn() capturing

[TeamPuzel]

When writing a List implementation this became a problem for functions like map filter or fold:

fn first(self, where: fn(T) -> Bool) -> Maybe[T]:
    for item in self:
        if where(item): return Maybe[T](item)
    return Maybe[T]()
    
fn first(self, where: fn(T) capturing -> Bool) -> Maybe[T]:
    for item in self:
        if where(item): return Maybe[T](item)
    return Maybe[T]()

I need to copy and paste every method for every combination of function parameters which is hard to maintain. I would expect all functions to work as parameters when capturing is present which turned out not to be true.

[lattner]

@laszlokindrat FYI we need to build support for subtyping of function types

[Moosems]

I would love to see the type() function come to python along with the Pipe operator (|) such that overloading doesn't create a bunch of boilerplate 😄.

[Moosems]

Much better to overload in scenarios where the code will be completely different and allow pipes and type() for areas where the code will remain relatively similar and one might just have to type cast.