Run assertions as tests

This adresses #48: At testing time, `examples/Arithmetics.lpts` is read, and each assertion in that file is treated as a separate test case. Try it by changing one of the assertions so that it fails and running `cabal test` or `dist/build/tests/tests`.

I see two problems with this code: (1) The implementation is insane. (2) The reaction to errors in file `Arithmetics.lpts` but outside any assertion is not optimal.

Comments?