Add methods to UseDeclarationStatementNode to determine if it imports a class, function, or constant (PHP 5.6)

[phenaproxima]

I propose something like this:

// These all return boolean
$use->importsClass();
$use->importsClass('\My\Awesome\Class');
$use->importsFunction();
$use->importsFunction('\pants_change');
$use->importsConstant();
$use->importsConstant('\MyNS\FOOBAR');

[grom358]

Merged.