extract some shared init logic

[Rich-Harris]

Some low-hanging fruit — rather than repeating this._handlers = Object.create(null) etc, we can just move that out into a shared init function. We could be more aggressive if we didn't mind having this._slotted and this.refs on all components, whether or not they're used (might mean modest savings on large apps that used those things, at the expense of ones that didn't) — for now, sticking to more unambiguous wins.