To keep the compilation firewall effect, the implementations
are hidden behind a PIMPL. In this case it is probably not
worth it, but we can inline it later if needed.
Doing this removes `<map>` from the include set of the base reporter
interface, and thus from bunch more TUs. This provides about 1.5%
improvements in the debug build of the static library, and 1% in
release build.
The naming scheme is simple, to include all matchers, include header
`catch2/matchers/catch_matchers_all.hpp`. To include **everything**,
include `catch2/catch_all.hpp`.
