Introduce compile time toggle to remove Matchers from TU

The toggle is `CATCH_CONFIG_DISABLE_MATCHERS` and the only use is
to speed up compilation of small TUs. For large ones it is likely
insignificant, because the speed up is constant relative to
number of tests/assertions in TU.

include/internal/catch_objc.hpp

@@ -101,6 +101,8 @@ namespace Catch {
         return noTestMethods;
     }
 
+#if defined(CATCH_CONFIG_DISABLE_MATCHERS)
+
     namespace Matchers {
         namespace Impl {
         namespace NSStringMatchers {
@@ -189,6 +191,8 @@ namespace Catch {
 
     using namespace Matchers;
 
+#endif // CATCH_CONFIG_DISABLE_MATCHERS
+
 } // namespace Catch
 
 ///////////////////////////////////////////////////////////////////////////////