Update and bug fixes for alignment

[cschreib]

List of changes in this PR:

• Fixed enable_observer_from_this* with policies that do not guarantee the existence of a block (i.e., custom policies that are not the default unique or sealed).
• Fixed leaks for non-sealed pointers, when allocation of the control block fails in the constructor or in reset().
• Fixed make_observable*() not aware of alignment constraints.
• Added early static_assert to catch misuse and generate clearer error messages.
• Various typos fixed, and documentation improved.
• Speed benchmarks updated.
• Size benchmarks updated.

[codecov-commenter]

Codecov Report

Merging #15 (bd82372) into main (61b47d5) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #15   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          299       317   +18     
=========================================
+ Hits           299       317   +18     

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

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