Some restructuring

[sbeyer]

Restructure bandit a little. Many classes are put into separate namespaces instead of populating the bandit::detail namespace. These new namespaces are designed in a consistent way, for example, instead of bandit::detail::listener and bandit::detail::dots_reporter (that implements listener), we now have bandit::reporter::interface and bandit::reporter::dots.

While at it

• the default formatter is renamed to posix,
• colorizer.h is moved out of the reporters subdirectory,
• test_run_summary is changed to a static class summary,
• the voidfunc_t type definition is removed,
• all typedefs are now implemented using the using keyword,
• get rid of useless bandit:: prefixes in spec sources.

[codecov-io]

Codecov Report

Merging #118 into master will increase coverage by 0.05%. The diff coverage is 87.63%.

@@            Coverage Diff             @@
##           master     #118      +/-   ##
==========================================
+ Coverage   89.64%   89.69%   +0.05%     
==========================================
  Files          30       31       +1     
  Lines         869      864       -5     
  Branches       73       73              
==========================================
- Hits          779      775       -4     
+ Misses         90       89       -1

Impacted Files	Coverage Δ
bandit/run_policies/filter_chain.h	100% <ø> (ø)
bandit/colorizers/colorizer.h	100% <ø> (ø)
bandit/registration/spec_registry.h	100% <ø> (ø)	:arrow_up:
bandit/reporters/spec.h	0% <0%> (ø)
bandit/reporters/interface.h	100% <100%> (ø)
bandit/grammar.h	98.63% <100%> (ø)	:arrow_up:
bandit/adapters.h	100% <100%> (ø)
bandit/run_policies/never.h	100% <100%> (ø)
bandit/run_policies/always.h	100% <100%> (ø)
bandit/failure_formatters/interface.h	100% <100%> (ø)
... and 29 more

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