Type translation of delegate types

[Jand42]

Part of #820

In WebSharper 4.0, delegates translate to JS funcions, with a few extra fields: $Func for the non-bound version, and $Target for the target object. Multi-cast delegates have an $Invokes which holds the invocation list as an array. All of these are treated as optional, so that any JS function works against WS-generated code that takes delegates.

Simply translating delegate types to their function signatures should be viable for a first-pass solution. So for example Action<int> becomes number => void. And then later it is possible to switch to generate named interfaces for them, like

interface Action<T0> extends WebSharper.MulticastDelegate {
  (a:T0): void;
}

• [x] Translated to lambda type
• [ ] Translated to named type

[Jand42]

Now translated to plain lambda signatures