PropertySpy property wrapper

[younata]

Resolves #7

Adds a new property wrapper for spying on and faking properties.

This is meant to be used like:

struct SomeType {
    @PropertySpy(2)
    var intValue: Int

    @PropertySpy(ConcreteImplementationOfSomeProtocol(), as: { $0 })
    var protocolValue: SomeProtocol
}

And then read like:

let instance = SomeType()
instance.$intValue // Spy<Void, Int>
instance.$protocolValue // Spy<Void, ConcreteImplementationOfSomeProtocol>

There is also a SettablePropertySpy, which works similarly, though the projectedValue (the $intValue or $protocolValue in the previous example) is a struct containing a getter and setter properties, which are spies recording calls to the property's get and set closures.

Checklist - While not every PR needs it, new features should consider this list:

• Does this have tests?
• Does this have documentation?
• Does this break the public API (Requires major version bump)?
• Is this a new feature (Requires minor version bump)?