add the 'status_code' function for getting all status codes as hash

[oalders]

This is a cherry pick and squash of #34 (2016), which in turn is a rework of #4 (2013).

It would be great to merge this before it turns 10 in February. 😢

Closes #34.

[codecov]

Codecov Report

Base: 94.57% // Head: 94.58% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (e183065) compared to base (3fdbbcb).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #194   +/-   ##
=======================================
  Coverage   94.57%   94.58%           
=======================================
  Files          10       10           
  Lines        1254     1255    +1     
  Branches      349      349           
=======================================
+ Hits         1186     1187    +1     
  Misses         14       14           
  Partials       54       54           

Impacted Files	Coverage Δ
lib/HTTP/Status.pm	92.85% <100.00%> (+0.54%)	⬆️

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