Checkbox from relationship

This PR adds support for repopulating checkboxes from Eloquent relationships/`Collection`s.

After doing #408 and this PR, it seems like support for this would better exist in `Populator::get()`, but I tried that and failed due to Select and Checkboxes differing in how they handle `Former::Populate` values in their constructors and `Checkable::isChecked()`/`Select::render()`.