#75 Make shared-implementation-objects thread-safe

[Andrei Kapustin]

CARL contains a lot of classes (aka strings, arrays, hash sets, etc.) which use reference-counted shared implementation to reduce assignment and copy overhead. This is currently not thread safe. Ti fix it, the "increment reference counter", "decrement reference counter" and "ensure nonshared implementation" helper methods of each such class must be converted into a monitor (aka synchronized).