omit, pick and partial should return schema's type

[rozaxe]

When passing a schema to omit, pick or partial, the resulting schema is always a struct object.()

It would be great to return the schema's type so that when passing a type(), the resulting schema is also a type().

Example:

const TodoSchema = type({
  id: string(),
  message: string()
})

const CreateTodoSchema = omit(TodoSchema, ['id'])
// CreateTodoSchema's is a struct 'object()' while it would great to keep it as a 'type' because it's input is.

I am willing to make the relevant PR if you want.