Support musl targets

[colinsmetz]

Hello !

First of all, thank you for that project, it is really nice :)

However, to be able to use it, we would need to have the -musl builds which are missing. I see that they have been removed in this commit. I'm not sure if there is a specific reason besides the fact that the compilation could not work without the appropriate options.

In any case, if you agree to add it, this PR should make it work (it did work on our fork).

I have also added the Cross.toml file, which is mentioned in the Precompilation guide. I am not sure the RUSTLER_NIF_VERSION is actually used without that file.

[maartenvanvliet]

Hi, thank you!!

I left out musl because of some other rustler projects I worked on where it didn't work in CI, so I didn't want to put in the effort. But it's not an issue here :)

There are still parts of SQL missing in the rust->elixir conversion so be mindful of that when using the library.

[colinsmetz]

Thanks for accepting the PR!

There are still parts of SQL missing in the rust->elixir conversion so be mindful of that when using the library.

Sure, I have seen that. We only need basic operations for now so it's not a problem :)

We'd be happy to contribute other improvements in the future if we need more features you are open to other PRs.