move gtfs/time.hpp into dedicated date library

[MoKob]

• avoids pulling in general gtfs dependencies, wherever the time module is used
• prepares for integration of general date formats

[jqtrde]

Codecov Report

Merging #101 into master will not change coverage. The diff coverage is 90.9%.

@@           Coverage Diff           @@
##           master     #101   +/-   ##
=======================================
  Coverage   96.32%   96.32%           
=======================================
  Files         112      112           
  Lines        2204     2204           
  Branches      198      198           
=======================================
  Hits         2123     2123           
  Misses         12       12           
  Partials       69       69

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