Make null-state non-observable

[jbcoe]

Make default constructor private. Make null-state only accessible via a move. Replace operator bool with valueless_by_move.

[jbcoe]

Docs need updating to merge this but I want to see what the API and coverage stats look like.

[codecov[bot]]

Codecov Report

Merging #120 (edf3ea7) into main (8f42b54) will decrease coverage by 1.26%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #120      +/-   ##
==========================================
- Coverage   96.63%   95.36%   -1.27%     
==========================================
  Files           2        2              
  Lines         505      410      -95     
==========================================
- Hits          488      391      -97     
- Misses         17       19       +2     

Impacted Files	Coverage Δ
polymorphic_value.h	87.69% <100.00%> (-2.09%)	:arrow_down:
polymorphic_value_test.cpp	98.92% <100.00%> (-0.26%)	:arrow_down:

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[BengtGustafsson]

There is also valueless_by_exception as in variant. I didn't think about the moved from state. This even more suggests that disallowing empty state is futile. With SBO the moved from object is still available and can be returned unless the object is larger than the SBO buffer.

[jbcoe]

We are abandoning this approach due to undesirable semantics of optional types with pointer-like interfaces.