Add a `form` helper to the __meta object to guide form generation

[Pomax]

Specifically:

__meta = {
  form: [
    {
      heading: <string or false, default to false>,
      controls: {
        save: <string for saving the form>,
        cancel: <string for resetting>
      }
      collapsible: <boolean, default to false>,
      collapsed: <boolean, default to false>,
      descriptions: <boolean, default to true>,
      fields: [<one or more var names, in the order they should appear on the form>],
    }
  ]
}

• heading: if a string, inject as form fieldset heading
• controls: if specified, this will place the form save/cancel buttons in this section. The value must be an object with string save and cancel properties,
• collapsible: if true, inject enough markup to effect pure-CSS collapsing
• collapsed: if true, and generate "pre-collapsed" markup. Note that collapsed: true implies collapsible: true, even if that field is present.
• descriptions: if note false, show field descriptions with the form control for them
• fields: the list of model fields that should be in this field set, in order or appearance.

Note that if there is a __meta.form, any fields not mentioned in the form array will not end up in the actual form markup.

[Pomax]

• [x] add support for __meta.form at all
• [x] add support for the form properties
• [x] uplift toForm
• [x] uplift toTable
• [x] uplift toTableRows
• [x] add at-load-time checking for models to see if their form has fields that don't exist.
• [ ] update the documentation to explain the form property