As far as I know, only a few users actually use it, but these changes
allow us to avoid including a surprising amount of code in the main
compilation path.
As part of `-Wdeprecated-copy-dtor` sweep, I noticed that Capturer's
copies are defaulted. Given that the class would likely break horribly
in the event of actual copy happening, they are now deleted.
This should improve the compilation times by decreasing the number
of TUs compiled, without making overly big TUs that would cause
problems with heavy-tailed compilation times.
There is one "combined TU" for the top level part, and each subpart,
except for Reporters, which currently do not have any trivial TUs.
Special member functions are now implicit, which should make them
both noexcept and constexpr. The `operator<<` has been made into
hidden friend as per best practices.
As Clara is no longer maintained as a separate project, the
implementation was moved to the internal subfolder of top-level
folder. This removes one folder and avoids potential user confusion.
Also simplified the convenience header checking script accordingly.
There are two reasons for this:
1) It is highly unlikely that someone has use for this header,
which has no customization points and only provides simplest
possible main, and cannot link the static library which also
provides a default main implementation.
2) It being a header was causing extra complications with
the convenience headers, and our checking script. This would either
require special handling in the checking script, or would break user's
of the main convenience header.
All in all, it is simpler and better in the long term to remove it,
than to fix its problems.
I do not think we need a safeguard against not including files in
CMake anymore, and as it is, it caused annoying false positive about
the default main implementation.
