* `operator[]` is constexpr
* `operator<<` and `operator+=` are hidden friends

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.