`assert.h` does nothing when the code is compiled in release
mode. That's not what we want :-). Let's reimplement `assert` as
`wasmer_assert`, which is close to the original `assert` function.
Remove the `build.rs` hack to add helper functions that are used only
in our tests. Now the tests can use the `tests/wasmer_wasm.h` file,
which simplifies the test workflow: No need to hack the build script
or anything if we want to add a helper function.
