Feature/sigma

[leissa]

• Removed:
  • .Arr, .Pack syntax for mutable arrays/packs
  • @ syntax for variables
• New syntax for mutable Pis/Sigmas:

  .Sigma Vec3: *, 3;                      // decl only
  .Sigma Vec3 = [x: F64, y: F64, z: F64]; // decl + def
  .lam foo v: Vec3 -> F64 = v#y;          // use field names
  .Pi Pipeline: □ = Π %compile.Phase -> Pipeline;

• Improved docs
• Already includes #207