3 implementations with benchmarks
Standardised on fastest impl for ObservableSubject.
Some hacky code but solid perf improvement.
Added log of perf changes.
Refactoring.
Much faster, but pre-tidy check-in.
More tidying.
Tidying
NotifyPropertyChangedLink now threadsafe.
Removed lots of messy 'spike' code.
Removed deprecated code.
Implemented NotifyPropertyChangedObservable.

Also refactored NotifyPropertyChangedValueObservable to use the
IObserver instance pattern.
Tests for NotifyPropertyChangedObservable
Cleaning Up Query Language.
More NotifyPropertyChangedObservable support

Also code clean up.
Added UserDataOrDefault.
Implemented PropertyValueChangedObservable 2
More Impl.
Tidied up tests.
Given_When improvements.