Make parser struct participate in Ruby's Garbage Collection

Make parser struct participate in Ruby's Garbage Collection

Instead of having individual str_t and ary_t members participate
in Ruby's mark-and-sweep Garbage Collection, put the parser
struct on the stack and make the parser participate; it will
be responsible for cleaning up its own member resources when
it falls out of scope.

Signed-off-by: Wincent Colaiuta <win@wincent.com>