Add manual promotion to arcsine, beta, and Bernoulli distributions

[mborland]

This is an extension to #1294 since it looks like most of the distributions have functions where manual promotion needs to be applied to follow what the policy says. I tried to reduce the ugliness as much as possible, but the result is that the code is generally less readable than it is without this PR. Thoughts @jzmaddock and @WarrenWeckesser. If there's no real opposition I can continue plugging along with this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.47%. Comparing base (483f36c) to head (2852b4b).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1299      +/-   ##
===========================================
- Coverage    95.07%   94.47%   -0.60%     
===========================================
  Files          796      686     -110     
  Lines        67020    49438   -17582     
===========================================
- Hits         63717    46705   -17012     
+ Misses        3303     2733     -570     

Files with missing lines	Coverage Δ
include/boost/math/distributions/beta.hpp	77.27% <ø> (-8.16%)	⬇️
include/boost/math/policies/policy.hpp	95.83% <ø> (ø)

... and 138 files with indirect coverage changes

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