Added support for index by and changing selection set

[edefimov]

This PR adds two new specifications with features and resolves #155 :

1. Support for indexing results by some field:

Spec::andX(
  Speс::indexBy('thing');
);

2. Adding fields to selection set

Spec::andX(
  Spec::addSelectionSet(['my_field', 'x.another_field'])
);

3. Replacing selection set with new one

Spec::andX(
  Spec::selectionSet(['my_field' , 'x.another_field'])
);

4. Aliasing selection set

Spec::andX(
  Spec::selectionSet(['my_field' => 'retVal' ])
);

Spec::andX(
  Spec::addSelectionSet(['my_field' => 'retVal' ])
);

[Nyholm]

Thank you. I will try to review it shortly.

[peter-gribanov]

Thanks for the work you've done.
Our plans have changed. We are currently working on features with operands #169 planned for major release 1.0. You can see the implementation of Selection concept here #172.
I can merge your feature into a minor release, but it will become obsolete with a major release. I would not want that. Sorry.

If you move the IndexBy query modifier to a separate PR, i can merge it.

[peter-gribanov]

Now you can using select and addSelect in specification:

$spec = Spec::andX(
    Spec::select('my_field'),
    Spec::addSelect('another_field')
)

$spec = Spec::andX(
    Spec::addSelect(Spec::field('another_field', $myDqlAlias))
)

$spec = Spec::andX(
    Spec::select(Spec::selectAs('another_field', 'my_field'))
)

$spec = Spec::andX(
    Spec::select(Spec::selectAs(Spec::value($myValue), 'my_field'))
)

[peter-gribanov]

@edefimov will you move the query modifier IndexBy to a separate PR? I would want to keep the authorship of this feature for you.

[edefimov]

yep, I will do it

[edefimov]

Separate PR with IndexBy feature: #192