Use configurable metadata for model

Instead of hardcoding properties and methods, use a schema to generate
them. This makes the code significantly dry·er at the cost of some
slightly obtuse metaprogramming.

The schema format is somewhat minimal and makes some assumptions; this
is not intended as a generalist O·W·L processor or anything of the
sort.

Previously, `TagSystem::Tag` returned a bound class constructor. With
this commit, it instead returns a (manually‐defined) constructor
function which effectively does the partial application in its function
body. Each constructor has its own unique prototype object (inheriting
from `Tag.prototype`) and its own static methods (bound forms of `Tag`
static methods). Constructors themselves inherit from
`Function.prototype` (for now).