Handle struct types more conveniently

[samestep]

Consider this example:

import { Real, Vec, fn, vec } from "rose";

const foo = fn([Real, Real], Real, (x, y) => {
  const z = f(x, y);
  const w = g(x, z);
  const u = h(z);
  const v = α(w, u);
  return v;
});

const bar = fn(
  [Vec(n, { x: Real, y: Real })],
  Vec(n, Real),
  (vals) => vec(n, Real, (i) => {
    const { x, y } = vals[i];
    return f(x, y);
  })
);

The x and y variables inside the body of the vec both get typed as symbol, when they should be typed as Real, causing this error in the call to f:

Argument of type 'symbol' is not assignable to parameter of type 'Real'.

If you add an as const after the { x: Real, y: Real } type, the correct behavior occurs, but this is ugly. There should be a way to handle these sorts of types without the user having to write as const.