Add unsafe dyncall support

[filmor]

This adds support for dynamic resource calls (https://www.erlang.org/doc/apps/erts/erl_nif#enif_dynamic_resource_call). The feature is inherently unsafe (exchanging pointers to mutable data between different dynamic libraries), though we might be able to eventually provide a stable API on top of it.

Changes to other parts:

• Exposes all of rustler_sys under rustler::sys s.t. it doesn't have to be explicitly referenced
• Adds helper function to compile tests and nif stubs only for particular NIF versions
• Ensures that RustlerTest can be loaded at startup

[filmor]

For reference, the error on Windows is

Oddly enough, this is during compilation (maybe because Elixir loads compiled artifacts?), not at runtime. I'll have to investigate a bit more.

/EDIT: The crash happens during the on-load callback, maybe there is something wrong with our spec of init_resource?

[filmor]

The issue described above is fixed with #643