Move all use of IResultCapture in AssertionHandler into the cpp file

This has two advantages

1) Removing the include of `catch_interfaces_capture.hpp` from the
   header and potentially allowing further pruning later.
2) Providing small-but-measurable 1-2% speedup for assertions.

src/catch2/internal/catch_assertion_handler.cpp

@@ -6,6 +6,7 @@
 
 // SPDX-License-Identifier: BSL-1.0
 #include <catch2/internal/catch_assertion_handler.hpp>
+#include <catch2/interfaces/catch_interfaces_capture.hpp>
 #include <catch2/interfaces/catch_interfaces_config.hpp>
 #include <catch2/internal/catch_context.hpp>
 #include <catch2/internal/catch_debugger.hpp>
@@ -14,6 +15,10 @@
 
 namespace Catch {
 
+    void AssertionHandler::finishIncomplete() {
+        m_resultCapture.handleIncomplete( m_assertionInfo );
+    }
+
     AssertionHandler::AssertionHandler
         (   StringRef macroName,
             SourceLineInfo const& lineInfo,

src/catch2/internal/catch_assertion_handler.hpp

@@ -10,12 +10,13 @@
 
 #include <catch2/catch_assertion_info.hpp>
 #include <catch2/internal/catch_decomposer.hpp>
-#include <catch2/interfaces/catch_interfaces_capture.hpp>
 
 #include <string>
 
 namespace Catch {
 
+    class IResultCapture;
+
     struct AssertionReaction {
         bool shouldDebugBreak = false;
         bool shouldThrow = false;
@@ -28,6 +29,8 @@ namespace Catch {
         bool m_completed = false;
         IResultCapture& m_resultCapture;
 
+        void finishIncomplete();
+
     public:
         AssertionHandler
             (   StringRef macroName,
@@ -35,9 +38,9 @@ namespace Catch {
                 StringRef capturedExpression,
                 ResultDisposition::Flags resultDisposition );
         ~AssertionHandler() {
-            if ( !m_completed ) {
-                m_resultCapture.handleIncomplete( m_assertionInfo );
-            }
+            // We want the common fast path inlinable, and the virtual
+            // dispatch in a function in single TU.
+            if ( !m_completed ) { finishIncomplete(); }
         }