Default interface should have no name, instead of copying "Constructor convention"

Right now, the default interface has the module name:

module multiply_add {
  interface multiply_add: int a, int b, int c -> int o
  /*...*/
}

Instead, we should have it be the abscence of a name:

module multiply_add {
  interface: int a, int b, int c -> int o
  /*...*/
}

Or further still, for purely functional modules:

module multiply_add : int a, int b, int c -> int o {
  /*...*/
}