Make typing functions using protocols more ergonomic

[evhub]

Should compile

def func[T <: (+) & (*)](x: T, y: T) -> T = x + x * y

to the equivalent of

def func[T <: ProtocolIntersection[SupportsAdd, SupportsMul]](x: T, y: T) -> T = x + x * y

where SupportsAdd is some special Coconut typing.Protocol for supporting __add__ and ProtocolIntersection is from here.

[evhub]

Since & is valid Python, we'll probably need to use a custom operator for protocol intersection, which will probably be &:.

[evhub]

Blocked on klausweiss/typing-protocol-intersection#4.

I'm just going to implement this with inheritance rather than ProtocolIntersection.