v3.0.0-preview3

--- Changes compared to preview 2

* Incorporated v2 changes up to v2.13.2
* Some runtime performance improvements
* The static library compiles 20-30% faster
* The overhead of including catch_test_macros.hpp is 25-30% smaller
* Amalgamated, 1 .hpp + 1 .cpp, distribution is provided
* Reporter interface has slightly changed
* Listener base renamed
* Rewritten contributing documentation
  * Primary development is now on `devel` branch
  * v3 is now the primary development version
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