Add #initFixture macro

[liamnichols]

Turn this:

fixture.register(Item.self) { values in
    Item(
        title: try values.get("title"),
        owner: try values.get("owner")
    )
}

Into this:

fixture.register(Item.self) { values in
    #initFixture(with: values, using: Item.init(title:owner:))
}

Used internally by @ProvideFixture, but also we can probably also introduce another new macro:

class MyTests: XCTestCase {
    let fixture = Fixture()

    override func setUp() {
        super.setUp()
        
        #register(Item.init(title:owner:), in: fixture)
    }
    
    // ...    
}

But ☝️ might be redundant because we could also leverage unapplied method references in @ProvideFixture(using: User.init(foo:bar:)) on extensions as a way to solve #13 at the same time. Attaching extension macros to extensions will not be possible in Swift 5.9

[liamnichols]

Using #initFixture inside closures is currently blocked by swiftlang/swift#67102