Remove fwd.hpp

[mborland]

Closes: #113

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.5%. Comparing base (23ae89a) to head (1e5a7a5).
⚠️ Report is 6 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop    #114   +/-   ##
=======================================
  Coverage     98.5%   98.5%           
=======================================
  Files          104     104           
  Lines         7119    7119           
  Branches      2634    2634           
=======================================
  Hits          7011    7011           
  Misses          42      42           
  Partials        66      66           

Files with missing lines	Coverage Δ
include/boost/safe_numbers/detail/type_traits.hpp	100.0% <ø> (ø)
...ost/safe_numbers/detail/unsigned_integer_basis.hpp	95.5% <ø> (ø)

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