RxBoomerang and RxBoomerangTest pods

This release introduce RxBoomerang and RxBoomerangTest specs for Cocoapods for a better support in test environments.
Support for macOS and watchOS is back (at ViewModel and compilation level)

Breaking Changes

• Removed func elementSize(at indexPath: IndexPath) -> ElementSize? in ListViewModel.

"Duality" between this method and func elementSize(at indexPath: IndexPath, type: String?) -> ElementSize? has caused some issues in normal usage in projects when overriding. We decided to keep a single method. This breaking change shouldn't have any impact on projects.

New stuff

• WithPropertyAssignment now works with both classes and structs, and includes a closure method to quickly edit object properties upon creation
  Example: UILabel().with { $0.text = "hey!" }
• Introducing RxBoomerang pod, which should be preferred over Boomerang/RxSwift. This change should allow developers to quickly switch between SPM and Cocoapods without impact on their existing sources.

As of today, using pod Boomerang/RxSwift in Podfile requires to import Boomerang only even for Rx extensions. SPM requires a double import (Boomerang + RxBoomerang). With this change we keep the two approaches in sync.

• Introducing RxBoomerangTest, a test framework to use in test targets, helping testing process of RxListViewModels and RxNavigationViewModels
• Support for MacOS and Watchos targets

There's no real added value at the moment, as we're missing support for AppKit and WatchKit extensions like we have on UIKit. Support is added to simply allow complex projects to link Boomerang in sub-frameworks.

• RxStateMachine: a state management helper built for tests based on ComposableArchitecture concepts (credits: @jrBordet)
• Some inline documentation
• ... this changelog :)

Fixes

• LICENSE.md is now properly recognized as MIT by automatic parsers.