Add macro for types-only schema

[westonpace]

It should be able to construct schemas with a syntax either like...

let schema = types_schema!({
  "": f32,
  "": {
    "": f64,
    "": f64,
  }
});

Or, preferably:

let schema = types_schema!({
  f32,
  {
    f64,
    f64
  }
});