Add XPath to collect parse tree nodes/subtrees.

XPath notation with exception that I introduce '!' negation operator. Samples:

    /prog/func,         -> all funcs under prog at root
    /prog/*,            -> all children of prog at root
     /*/func,            -> all func kids of any root node
    prog,               -> prog must be root node
    /prog,              -> prog must be root node
    /*,                 -> any root
    *,                  -> any root
    //ID,               -> any ID in tree 
    //expr/primary/ID,  -> any ID child of a primary under any expr
    //body//ID,         -> any ID under a body
    //'return',         -> any 'return' literal in tree
    //primary/*,        -> all kids of any primary 
    //func/*/stat,      -> all stat nodes grandkids of any func node
    /prog/func/'def',   -> all def literal kids of func kid of prog
    //stat/';',         -> all ';' under any stat node
    //expr/primary/!ID, -> anything but ID under primary under any expr node
    //expr/!primary,    -> anything but primary under any expr node
    //!*,               -> nothing anywhere
    /!*,                -> nothing at root