This allows us to move <stdexcept> out of the common path, and replace
it with just <exception>. The difference between these two headers is
~13k lines after preprocessing on libstdc++ (16k vs 3k) and ~17k lines
for MS's STL(33k vs 16k).

Note that this is only beneficial if no other stdlib header we use
includes <stdexcept>. AFAIK this is true for the newest MS's STL,
but I have no idea of the applicability for libstdc++ and libc++.

This means

* Adding new configuration toggle `CATCH_CONFIG_DISABLE_EXCEPTIONS`
and a best-guess configuration auto-checking for it.
* Adding new set of internal macros, `CATCH_TRY`, `CATCH_CATCH_ALL`
and `CATCH_CATCH_ANON` that can be used in place of regular `try`,
`catch(...)` and `catch(T const&)` respectively, while disappearing
when `CATCH_CONFIG_DISABLE_EXCEPTIONS` is enabled.
* Replacing all uses of `throw` with calls to `Catch::throw_exception`
customization point.
* Providing a default implementation for the above customization point
when `CATCH_CONFIG_DISABLE_EXCEPTIONS` is set.
* Letting users override this implementation with their own.
* Some minor changes and ifdefs all around to support the above