Add local variable with the “let” keyword.

A local variable is a contract variable, it does not represent a data of
the program (class attribute or method argument).

In Praspel:
    @requires let size: 1..5
          and arr: array([…], size);
In PHP
    $requires = $specification->getClause('requires');
    $requires->let['size']->in = realdom()->boundinteger(1, 5);
    $requires['arr']->in       = realdom()->array(…, $requires['size']);
Remove “xor” proposal.