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.

I do not know if checking the tracker name or the tracker's file
part of the location first would provide better results, but
in the common case, the line part of the location check should be
rather unique, because different `SECTION`s will have different
source lines where they are defined.

I also propagated this same check into `ITracker::findChild`,
because this significantly improves performance of section tracking
in Debug builds -> 10% in macro benchmark heavily focused on section
tracking. In Release build there is usually no difference, because
the inliner will inline `NameAndLoc::operator==` into `findChild`,
and then eliminate the redundant check. (If the inliner decides
against, then this still improves the performance on average).

`NameAndLocationRef` is pretty large type, so even in release build,
it is unlikely to be passed in registers. In addition to the fact
that some platforms currently do not allow passing even small types
in register (Windows ABI!!), it is better to pass it as a ref,
effectively passing around a pointer.

Now we delay allocating owning `NameAndLocation` instances until
we construct a new tracker (because a tracker's lifetime can be
significantly different from the underlying tracked-thing's name).

This saves 4239 allocations (436948 -> 432709) when running
`./tests/SelfTest -o /dev/null`, at some cost to code clarity
due to introducing a new ref type, `NameAndLocationRef`.