Adds a new n_USE_LEXICAL_VARIABLES node. This will make more sense after my next diff (see D14615).

I'm not 100% set on the name of this node, so feel free to suggest alternatives. I personally find the use of use in PHP somewhat confusing because it is a rather overloaded construct:

• It is used to alias/import namespaces (see http://php.net/manual/en/language.namespaces.importing.php).
• It is used to insert traits (see http://php.net/manual/en/language.oop5.traits.php).
• It is used for lexical variable scoping in anonymous functions (see http://php.net/manual/en/functions.anonymous.php).

For completeness, the following relationships are meant to be similar:

• n_ARRAY_LITERAL and n_ARRAY_VALUE_LIST
• n_LIST and n_ASSIGNMENT_LIST
• n_USE_LEXICAL_VARIABLES and n_LEXICAL_VARIABLE_LIST
• n_USE_LIST and n_USE

Added unit tests.