allow Symbol-named methods in virtual/durable objects

[warner]

What is the Problem Being Solved?

@erights noticed that virtual objects (defineKind and defineDurableKind) only accept string-named methods in their behavior records. He needs to use Symbol.asyncIterator to build a durable Notifier (#4567).

@FUDCo says this will be pretty easy to do once #2481 is complete.

Description of the Design

defineKindInternal needs to scan the behavior records with something like Reflect.keys or Object.getOwnPropertySymbols instead of / in addition to Object.keys / Object.getOwnPropertyNames, to get all the symbol-named properties too.

Security Considerations

Far/Remotable already have a set of rules about what is/isn't allowed as a property, and we want to be consistent with that, but we already use Far to create the Representatives so worst-case we're too restrictive instead of too liberal.

Test Plan

unit tests