Changed Reference Semantics

• Incompatible Change: Reverted the weakable reference semantics with regard
  to capturing. It's too unintuitive. You will have to invest the overhead
  about which closures capture what and why. The other semantics are too
  unpredictable.
• Incompatible Change: Removed the extra value of drop function. They can
  just as easily passed as upvalue.
• Change: Previous weakable references are now called hidden references,
  because they are automatically dereferenced on variable access.
• Incompatible Change: Strong references are not implicitly dereferenced
  on variable access anymore.
• Incompatible Change: iter $&&i, which was a bad idea, was reverted.