handle set after destroy, and move destroy into shared helpers

[Rich-Harris]

Fixes #744, by checking for this._destroyed before calling set. Though now that I think about it, replacing the set method with noop might be better... will make that change in a sec.

Anyway, I also moved the destroy method into the shared helpers. The only bit of component-specific stuff in the destroy method relates to the ondestroy lifecycle hook, and that can be dealt with as an event handler. Should shave a few bytes off apps with multiple components.

[Conduitry]

Might we also want to replace destroy with noop`? You could still get the dev warning by also doing

export function destroyDev(detach) {
	destroy.call(this, detach);
	this.destroy = () => console.warn('Component was already destroyed');
}

[Rich-Harris]

That's brilliant! Now we don't even need to keep track of this._destroyed.