Dynamically load Erlang's symbols on startup instead of linking them

[filmor]

Doing the "linking" explicitly using dlopen and dlsym has the advantage that

1. The rustc flags on macOS become unnecessary
2. We can add an additional main to list the NIFs without having to fake the symbols (as done in cargo-rustler #614)
3. Startup and calling would work the same way on all OSes, allowing us to ditch the build script (and thus speeding up the build)