Split catch_reporter_bases.hpp into two separate headers

Each of the two reporter bases now has its own header file, and
cpp file. Even though this adds another TU to the compilation,
the total CPU time taken by compilation is reduced by about 1%
for debug build and ~0.5% for optimized build of the main library.
(The improvement would be roughly doubles without splitting the TUs,
but the maintainability hit is not worth it.)

The code size of the static library build has also somewhat decreased.

Follow up: Introduce combined TU for reporters, and further split
apart the catch_reporter_streaming_base.hpp header into its
constituent parts, as it still contains a whole bunch of other stuff.

examples/210-Evt-EventListeners.cpp

@@ -6,7 +6,7 @@
 // 3. Test cases
 
 #include <catch2/catch_test_macros.hpp>
-#include <catch2/reporters/catch_reporter_bases.hpp>
+#include <catch2/reporters/catch_reporter_streaming_base.hpp>
 #include <catch2/catch_reporter_registrars.hpp>
 #include <catch2/catch_test_case_info.hpp>
 #include <iostream>