Add enable_observer_from_this and fix self assignment/swap

[cschreib]

This PR brings enable_observer_from_this, to create observer pointer from within a class, and fixes bugs related to self assignment and self swap. It also fixes a mismatch between new[] and delete for the sealed pointer, which was reported by valgrind.

[codecov-commenter]

Codecov Report

Merging #3 (91ef2e1) into main (c8acb1f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main        #3   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          268       275    +7     
=========================================
+ Hits           268       275    +7     

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

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