First-class traits

[chrisaycock]

Recursive functions require that the traits (and return type) be explicitly listed:

func factorial(n: Int64) pure, transform, linear -> Int64:
  if n == 0:
    return 1
  end
  return n * factorial(n - 1)
end

Traits will need to be identifiers the same way that types currently are.

[chrisaycock]

The HIR will likely need TraitsKind and ModeKind to complement TypeKind. This will eliminate the need for codegen's direct_repr in most cases since higher-order items are preferable.