Implement IBindable interfaces for get-only support

[smoogipoo]

Doesn't change existing code, but will now allow us to expose bindables as IBindable and IBindable<T> with get-only access, disallowing external objects from setting values.

Bindables must be exposed as the same interface to be able to bind to each other, or must use potentially unsafe casting to bind to interface-types. The following compile-time behaviour is observed:

IBindable bindable1 = new Bindable<int>();
IBindable bindable2 = new Bindable<int>();
IBindable<int> bindable3 = new Bindable<int>();
IBindable<int> bindable4 = new Bindable<int>();
Bindable<int> bindable5 = new Bindable<int>();
Bindable<int> bindable6 = new Bindable<int>();

bindable1.BindTo(bindable2); // Allowed
bindable1.BindTo(bindable3); // Not allowed (not assignable)
bindable1.BindTo(bindable5); // Allowed

bindable3.BindTo(bindable1); // Not allowed (not assignable)
bindable3.BindTo(bindable4); // Allowed
bindable3.BindTo(bindable6); // Allowed

bindable5.BindTo(bindable1); // Not allowed (not assignable)
bindable5.BindTo(bindable3); // Not allowed (not assignable)
bindable5.BindTo(bindable6); // Allowed

bindable5.BindTo((Bindable<int>)bindable1); // Allowed, but unsafe and may lead to InvalidCastExceptions
bindable5.BindTo((Bindable<int>)bindable3); // Allowed, but unsafe and may lead to InvalidCastExceptions

bindable2 = bindable1.GetBoundCopy(); // Allowed
bindable2 = bindable3.GetBoundCopy(); // Not allowed (cannot convert)
bindable2 = bindable5.GetBoundCopy(); // Allowed

bindable4 = bindable1.GetBoundCopy(); // Not allowed (cannot convert)
bindable4 = bindable3.GetBoundCopy(); // Allowed
bindable4 = bindable5.GetBoundCopy(); // Allowed

bindable6 = bindable1.GetBoundCopy(); // Not allowed (cannot convert)
bindable6 = bindable3.GetBoundCopy(); // Not allowed (cannot convert)
bindable6 = bindable5.GetBoundCopy(); // Allowed