Clara cleanups

* Clara is now split between a header and a cpp file.
* Removed the deprecated `+` and `+=` operators for composing
  a parser.
* Renamed `clara` and `detail` namespaces to be inline with the
rest of Catch2 (they are now `Clara` and `Detail` respectively).
* Taken most of user-exposed types out of the `Detail` namespace
completely (instead of using `using` directives to bring them into
the outer namespace).

src/CMakeLists.txt

@@ -44,7 +44,6 @@ set(INTERNAL_HEADERS
     ${SOURCES_DIR}/catch_assertion_result.hpp
     ${SOURCES_DIR}/internal/catch_test_macro_impl.hpp
     ${SOURCES_DIR}/internal/catch_clara.hpp
-    ${SOURCES_DIR}/internal/catch_clara_upstream.hpp
     ${SOURCES_DIR}/internal/catch_commandline.hpp
     ${SOURCES_DIR}/internal/catch_common.hpp
     ${SOURCES_DIR}/internal/catch_compiler_capabilities.hpp
@@ -138,6 +137,7 @@ set(IMPL_SOURCES
     ${SOURCES_DIR}/internal/catch_assertion_handler.cpp
     ${SOURCES_DIR}/catch_assertion_result.cpp
     ${SOURCES_DIR}/matchers/internal/catch_matchers_combined_tu.cpp
+    ${SOURCES_DIR}/internal/catch_clara.cpp
     ${SOURCES_DIR}/internal/catch_commandline.cpp
     ${SOURCES_DIR}/internal/catch_common.cpp
     ${SOURCES_DIR}/catch_config.cpp