Fixes #468

We could implement this by simply holding a List<string> state in the specification. However, the memory overhead of a non-empty list is 72 bytes (on x64). Considering that in 99% of cases, users will add a single tag, the memory overhead is unacceptable.

Instead, we'll create and use OneOrMany struct implementation, which has a single object? state (spending only 8 bytes for the reference). The state will represent a string or List<string depending on the situation.