There is no good reason for these to be std::strings, as these
are just (optional) constants for nice user output. This ends up
reducing the allocations significantly.
When measuring allocations when running no tests, the changes are
`tests/SelfTest` 9213 -> 7705
`tests/ExtraTests/NoTests` 3723 -> 2215
Most of the changes are completely pointless renaming of constructor
arguments so that they do not use the same name as the type members,
but 🤷Closes#2015
Anchoring the vtables does 2 things
1) Fixes some instances of `-Wweak-vtables`
2) Decreases code size and linker pressure
However, there are still some unanchored ones, and thus we have
to keep suppressing `-Wweak-vtables` warning for Clang.
* Clara is now split between a header and a cpp file.
* Removed the deprecated `+` and `+=` operators for composing
a parser.
* Renamed `clara` and `detail` namespaces to be inline with the
rest of Catch2 (they are now `Clara` and `Detail` respectively).
* Taken most of user-exposed types out of the `Detail` namespace
completely (instead of using `using` directives to bring them into
the outer namespace).
