Decide on syntax for mesh! macro

[LukasKalbertodt]

We have two arrays: vertices and faces. For each element we can specify additional properties after the "core info". The question is how to specify multiple properties. Potential syntaxes:

• As tuple:

  vertices: [
      v0: (prop0, prop1, prop2), // multiple
      v1 : (prop0), // single
  ]
  

  Simple, but feels like too many parenthesis, especially for a single element.

• As tuple with special casing single value:

  vertices: [
      v0: (prop0, prop1, prop2), // multiple
      v1 : prop0, // single
  ]
  

  Problem: if someone tries to specify a position as tuple (after all (f32, f32, f32) implements Pos3Like), strange errors will occur (it will be parsed as three properties instead of one). Generally, it feels like too much magic IMO.

• Comma separated, with semicolon seperated elements

  vertices: [
      v0: prop0, prop1, prop2; // multiple
      v1 : prop0; // single
  ]
  

  This diverges from the normal Rust "list like syntax": [a, b, c].