* 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.

The license file was renamed with 6a502cc2f5fedd59b3495b58708f0d6d987ed9e1

This might become potentially useful in the future, when we want
to provide the ability to forward jump generators, to be able to
simply reproduce specific input to a test.

I am not yet sure how it would work, but the basic idea is that
it could be similar to the `-c` switch for selecting specific
`SECTION` in a test case.

This should also be done for test files, but that has lower priority.

This commit also strips the old copyright comment header in touched
files, as those will also be replaced with a more standardized and
machine-friendly version.

Doing some benchmarking with ClangBuildAnalyzer suggests that
compiling Catch2's `SelfTest` spends 10% of the time instantiating
`std::unique_ptr` for some interface types required for registering
and running tests.

The lesser compilation overhead of `Catch::Detail::unique_ptr` should
significantly reduce that time.

The compiled implementation was also changed to use the custom impl,
to avoid having to convert between using `std::unique_ptr` and
`Catch::Detail::unique_ptr`. This will likely also improve the compile
times of the implementation, but that is less important than improving
compilation times of the user's TUs with tests.