Make RuntimeBuilder agnostic to library extension

[Wodann]

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• setup repository on your local machine and make sure all tests pass (cargo test)
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• the Mun Runtime receives RuntimeOptions from the RuntimeBuilder in order to load a shared library. At the moment this library path is not platform agnostic (i.e. internally determines whether it is dealing with a *.so OR *.dll OR *.dylib file). This results in code like this (a contrived example):

  fn runtime_unix() -> Runtime {
      RuntimeBuilder::new("voxygen/src/anim/character/test.so")
          .spawn()
          .expect("Failed to spawn Runtime");
  }
  
  fn runtime_windows() -> Runtime {
      RuntimeBuilder::new("voxygen/src/anim/character/test.dll")
          .spawn()
          .expect("Failed to spawn Runtime");
  }

  Per @sonovice's submission, we want to instead generate libraries with a custom extension: *.munlib:

  fn runtime() -> Runtime {
      RuntimeBuilder::new("voxygen/src/anim/character/test.munlib")
          .spawn()
          .expect("Failed to spawn Runtime");
  }

  To implement this, you need to:
  • replace *.so / *.dll / *.dylib extensions with *.munlib in the Mun Compiler when writing to file
  • check that RuntimeBuilder::new is allowed to receive any path (both with and without extension)
• start a Pull Request. Set description to closes #72. If this is your first PR, welcome 🎉 😄

[legendiguess]

I would like to contribute to this issue 👀