Not being able to filter generators to specific element has been regularly
causing problems. It was possible to use a dynamic section to run tests
for specific element in a generator, at least if the element had a nice
string representation, but the test case would still run once per element
in the generator.

With this change, it is possible to have the generator return only one
specific element, and do so based on the index, rather than the string
representation of the element. This enables simple debugging of tests
that fail for specific generator element.

* Removed `IGeneratorTracker::hasGenerator`, as it was never used.
* Removed `IGeneratorTracker::setGenerator`, as actually using it
  after the tracker was used would lead to inconsistent behaviour.
  Instead, the tracker is given the generator it guards during
  construction.

This will be useful later to implement warning on infinitely
running `GENERATE` expressions.

Closes #2954
Closes #3010

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.

Since the change to make the message macros thread-safe, and thus
thread-local, there is no need to handle messages through instance
of `RunContext`.

Together with the previous change, this means that `ScopedMessage`
only needs to keep around a single unsigned int.