Add a helper for getting the StringId of a DexString

[vinceTheProgrammer]

It looks like the Strings struct has a get_id() method, which perhaps is what I'm looking for, but it's not exposed, maybe for a good reason. A helper for smooth brains like myself to safely get the StringId of a DexString would be helpful.

[vinceTheProgrammer]

Found a solution for my particular use case, so no rush cause it's not holding back my project anymore. That being said, it could still be a helpful helper to consider.

In my case I needed to populate a Vec of my own String type but with indices exactly matching the StringId of each DexString.

When iterating the .strings(), each DexString does not let me get the StringId from it, so I couldn't put it into the Vec with confidence that the index matches its StringId. But taking a look at the StringsIter struct that the strings() function returns, it looks like the .next() function calls the cloned Strings.cache.get() which takes StringId, so if I understand correctly, it should be guaranteed to iterate in order of StringId ✅, which is good enough for my specific use case.