FallbackSymbol added

This is the closest we will get to implementing a concept similar to symbols
in pre-ES6. The intent is that, in an ES5 environment, the caller should
ensure that the object receiving this key will mark it as non-enumerable.
Otherwise, we're out of luck.

The symbol string is pseduo-randomly generated with an attempt to reduce the
likelihood of field collisions and malicious Math.{floor,random} overwrites
(so long as they are clean at the time of loading the module).
util/Symbol added
ClassBuilder#build now only making one getMeta call
Corrected MemberBuilderValidator warning handler in class.js
Class ___$$meta$$ moved into private symbol field
Visibility object is now encapsulated in symbol key

Note that this patch temporarily breaks encapsulation via
ClassBuilder.___$$privsym$$ to expose necessary internal details to Trait;
this will be resolved.