The license file was renamed with 6a502cc2f5fedd59b3495b58708f0d6d987ed9e1

By not materializing the lower cased tags ahead of time, we
save allocations at the cost of worsened performance when comparing
two tags.

Since there are rarely many tags, and commonly they are not
compared even if present, this is almost always a win. The new
implementation also improves the robustness of the code
responsible for handling tags in a case-insensitive manner.

This is has been what it actually does for a long time, but it
has not been renamed earlier due to API compatibility.

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

This is not nearly all of them, because IWYU does not support the
way Catch2 manages includes -- it expects that non-system includes
are done using `#include "foo/bar/baz.hpp"`, while Catch2 uses
`<foo/bar/baz.hpp>`. This causes trouble, because IWYU suggests
removing every single internal header, and then adding them again,
but using `""` in the include directive... the resulting suggestions
cannot be used without a lot of manual work, as they are largely
bogus.

For bonus points, IWYU also _loves_ to suggest kinda-random stdlib
headers for `size_t` and similar. Still, the resulting inclusion
graph is somewhat better than it was before.

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.

This is both a really big and a really small commit. It is small in
that it only contains renaming, moving and modification of include
directives caused by this.

It is really big in the obvious way of touching something like 200
files.

The new rules for naming files is simple: headers use the `.hpp`
extension. The rules for physical file layout is still kinda in
progress, but the basics are also simple:
 * Significant parts of functionality get their own subfolder
   * Benchmarking is in `catch2/benchmark`
   * Matchers are in `catch2/matchers`
   * Generators are in `catch2/generators`
   * Reporters are in `catch2/reporters`
   * Baseline testing facilities are in `catch2/`
 * Various top level folders also contain `internal` subfolder,
   with files that users probably do not want to include directly,
   at least not until they have to write something like their own
   reporter.
    * The exact files in these subfolders is likely to change later
      on

Note that while some includes were cleaned up in this commit, it
is only the low hanging fruit and further cleanup using automatic
tooling will happen later.

Also note that various include guards, copyright notices and file
headers will also be standardized later, rather than in this commit.