v0.6.0

Main changes / breaking changes:

• Added basic_observable_ptr, basic_observer_ptr, basic_enable_observer_from_this and policies to fine tune their behavior and API.
• observable_unique_ptr, observable_sealed_ptr, observer_ptr are now defined as aliases to the above types with pre-configured policies, which are expected to be the most user-friendly / useful policies. But other choices of custom policies are viable and allowed.
• Breaking: enable_observer_from_this has been removed and is now replaced by enable_observer_from_this_unique (for observable_unique_ptr owners) and enable_observer_from_this_sealed (for observable_sealed_ptr owners).
• enable_observer_from_this_unique preserves the same API and behavior as the now defunct enable_observer_from_this, with the added benefit that calling observer_from_this() in constructors will return a valid, non-null pointer. The function is now guaranteed to always return a valid pointer, and is thus noexcept. However, the constructor of enable_observer_from_this_unique allocates, and thus cannot be noexcept.
• enable_observer_from_this_sealed requires the object T to take a control block as first argument of its constructor, to be forwarded to enable_observer_from_this_sealed. This enables calling observer_from_this() in the constructor. The function is now guaranteed to always return a valid pointer, and is thus noexcept.
• The memory footprint of the control block has been optimised.

Small changes and bug fixes:

• Improvements to the documentation.
• Added more compile-time tests.