vm: add isContext; prevent double-contextifying

Previously, calling `vm.createContext(o)` repeatedly on the same `o`
would cause new C++ `ContextifyContext`s to be created and stored on
`o`, while the previous resident went off into leaked-memory limbo.
Now, repeatedly trying to contextify a sandbox will do nothing after
the first time.

To detect this, an independently-useful `vm.isContext(sandbox)` export
was added.