Qualified Cursor Support

[mpyw]

This PR makes it possible to use qualified cursor.

$tags = Tag::find(1)->posts()->withPivot('id')
    ->lampager()
    ->forward()->limit(3)
    ->orderBy('posts.id')
    ->seekable()
    ->paginate(['posts.id' => 2]);

[coveralls]

Coverage remained the same at 100.0% when pulling c114838 on qualified-cursor into a063521 on master.

[chitoku-k]

The implementation looks fine but more tests can be added!

[mpyw]

self-LGTM

[chitoku-k]

What is wrapped value mentioned in the commit message?

[mpyw]

@chitoku-k

Wrapping means... alternatively "enclosed"

e.g.

• id --(wrap)--> `id`
• posts.id --(wrap)--> `posts`.`id`

orderBy() orderByDesc() can't accept already enclosed values.

[chitoku-k]

LGTM