(feat) Handler unsafe.

- Prepares `Furnace` for `Iron` integration
 - Adds `from_furnace`, `new`
 - Adds `iron.smelt`
 - Documents new methods on `Furnace`
 - Implements defaults for `IronFurnace`
- Implements `handle_request` (unsafely, to `transmute` the `ResponseWriter` to trait type (`Rs`)

This can be broken into two PRs if that is preferable (`furnace` and `handle_request`), but the latter depends on the former, so we would need to chain them either way.