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.

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