Inline the getResultCapture helper into header.

We still keep the error check in the function, but hide it in an
outlined function inside a .cpp file, to promote inlining of the
retrieval part.

In the future, we should explore two things

1) Skipping over the context retrieval here, allowing direct access.
   I currently do not see a way to do this while keeping the
   "greppability" of mutable vs immutable accesses that is there now,
   but it would help a lot when inlining is not enabled.
2) Removing the error check, to make the function trivially inlinable,
   and without branches.

**runtime difference**

| --------- | Debug | Release |
|:----------|------:|--------:|
| Slow path |  0.98 |    1.07 |
| Fast path |  1.04 |    1.08 |

We lost bit of performance on the assertion slow path in debug mode,
but together with the previous commit, it comes out at net zero.
For other combinations, we see 5-10% perf improvement across the
two commits.

src/catch2/internal/catch_run_context.cpp

@@ -809,13 +809,6 @@ namespace Catch {
         }
     }
 
-    IResultCapture& getResultCapture() {
-        if (auto* capture = getCurrentContext().getResultCapture())
-            return *capture;
-        else
-            CATCH_INTERNAL_ERROR("No result capture instance");
-    }
-
     void IResultCapture::pushScopedMessage( MessageInfo&& message ) {
         Detail::g_messages.push_back( CATCH_MOVE( message ) );
     }