closure support, ugly implementation :(
some fixes of closure bugs (continue patching)

  o new built-in `set!' function
  o Context#lookup_context now return nil instead of raising exception
    when a symbol is not found
  o Context#clear to clear symbol table of a context
  o bug found and marked for built-in `fluid-let'
  o closure rspec testcases
    * regular adder generator
    * y combinator!