I've added -Wextra as well as removed some of the -Wno-* flags and fixed issues in the code.

Also fixed the cmake build to put user-defined flags after default flags so that they can be overridden.

The idea behind the directory organization was that we might have official implementations for other languages in other top-level directories, as siblings to the c++ directory. It seems implausible that we'd use CMake as a meta-build-system over all of these.

Another reason for this change is that the release tarball actually contains only the c++ subdirectory. We probably want to include cmake files in the release.

CMake automatically patches the switch character.

With CAPNP_LITE=1, the command-line tools will not be built, and the
tests cannot be built without EXTERNAL_CAPNP=1 (BUILD_TESTING=0 needs to
be set if the tests are not desired).

* Move binaries into the bin directory and libs into the lib directory in the output
* Make sure the actual binary output file of capnp-tool is just capnp
* Remove need for deprecated cmake get_property function