Syms

Moving symbols from FReft to Ref.

* add Sorted symbols on Ref
Ref t s m
  = RMono [(Symbol, t)] s   
  |  RPoly   [(Symbol, t)] m

* remove FReft and replace it with simple Fixpoint.Reft

* remove f/add/dropSyms from Reftable

* remove refsymbs from CG State Monad