[RFC] structs with defaults

[aleksanderkatan]

Use case: some libraries might want to export functions with default parameters, a struct with default properties might be used to achieve this.

Example API:

const Schema = d.withDefaults(
  d.struct({
    a: d.vec3f,
    b: d.vec3f,
  }),
  {
    b: d.vec3f(1, 2, 3),
  },
);