Updating this package for new OTP

1. Inspect new nif version at https://github.com/erlang/otp/blob/master/erts/emulator/beam/erl_nif.h and add new version_opts() clause at https://github.com/rusterlium/erlang_nif-sys/blob/master/gen_api.erl#L33 .

2. Inspect new nif functions at https://github.com/erlang/otp/blob/master/erts/emulator/beam/erl_nif_api_funcs.h and add them to api_list() at https://github.com/rusterlium/erlang_nif-sys/blob/master/gen_api.erl#L324 . Add new datatypes to https://github.com/rusterlium/erlang_nif-sys/blob/master/src/erlang_nif_sys_api.rs as needed.

3. Add tests.

• Add Rust NIFs invoking some or all new nif api functions to https://github.com/rusterlium/erlang_nif-sys/blob/master/tests/testapp/crates/mynifmod/src/lib.rs .
• Add Erlang invocations of the Rust NIFs to https://github.com/rusterlium/erlang_nif-sys/blob/master/tests/testapp/mynifmod.erl .
• Run tests with cargo test (IIRC).
• Update https://github.com/rusterlium/erlang_nif-sys/blob/master/.travis.yml and https://github.com/rusterlium/erlang_nif-sys/blob/master/appveyor.yml with references to current Erlang version.