Add pointer casts

[cschreib]

Main changes:

• Added explicit casting constructors to observer_ptr
• Added static_pointer_cast, const_pointer_cast, and dynamic_pointer_cast for all pointer types

Small changes and bugfixes:

• Fixed explicit casting constructors for owner pointers leaking the control block when given a null pointer
• Fixed explicit casting constructor for sealed pointers forcing a T* as input, rather preserving the user's pointer type throughout. In theory this could have broken enable_observer_from_this in some edge cases (T* not inheriting from enable_observer_from_this, but user-supplied V* does). However in practice, the raw pointer must come from a sealed pointer, and will therefore already have its control block setup correctly. So this has no observable consequence.

[codecov-commenter]

Codecov Report

Merging #5 (6efeca0) into main (a1c175e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main        #5   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          285       329   +44     
=========================================
+ Hits           285       329   +44     

Impacted Files	Coverage Δ
include/oup/observable_unique_ptr.hpp	100.00% <100.00%> (ø)

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