Fix deprecated-copy warnings in polynomial.hpp

[Flamefire]

Explicitly define the copy/move constructor as the implicit copy constructor for 'reference' is deprecated because it has a user-declared copy assignment operator

I moved reference &operator^=(bool b) to the bottom to avoid it being inside the conversion operators, so they can be closer together making reasoning about them easier

I noticed that operator bool should likely be explicit

Alternative to #111 after #110 got closed even though considered correct by @mborland

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.79%. Comparing base (21b0544) to head (c6e66c5).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #166   +/-   ##
========================================
  Coverage    95.79%   95.79%           
========================================
  Files          117      117           
  Lines         7660     7660           
========================================
  Hits          7338     7338           
  Misses         322      322           

Files with missing lines	Coverage Δ
include/boost/random/detail/polynomial.hpp	96.93% <100.00%> (ø)

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[mborland]

Looks good. Thanks! Will merge once the CI finishes.