Now, the private symbol is, 1. it is SymbolImpl 2. and it is managed by VM's private symbol table This patch moves this private symbol flag to SymbolImpl itself. It allows us to define PrivateSymbols without interactiving VMs. VM's table is used for mapping between public and private symbols *in the parser*. It is more correct in the current use cases since we use `xxx.@iterator` for public @iterator symbol.