Previously, each warning suppression was self-contained, with its
own pair of `SUPPRESS_X_WARNING` and `UNSUPPRESS_X_WARNING` macros.
This had the obvious advantage of being self-containing, but it
also meant that if we needed to suppress more than one warning
in a single place, then we would manipulate the compiler's warning
state multiple times, even though logically we would only need one
layer.

The new way of suppressing warnings in macros is to push compiler's
warning state with `CATCH_INTERNAL_START_WARNINGS_SUPPRESSION` macro,
then disable whatever macros we need with the
`CATCH_INTERNAL_SUPPRESS_X_WARNINGS` macro, and then return to the
previous state using `CATCH_INTERNAL_STOP_WARNINGS_SUPPRESSION`.

This adds UNSCOPED_INFO macro, creating a log message that is stored
until the end of next assertion or the end of test case, whichever comes
first. These messages are not scoped locally, unlike messages created by
INFO macro.

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

This avoids having to call `strlen` to get the constant string's length
and thus should improve performance.