[C++] Optimize ATN deserialization

[jcking]

Improve ATN deserialization by avoiding unnecessary copies and right-sizing std::vector when the size is known ahead of time.

[jcking]

@mike-lischke

[mike-lischke]

I like the clean up of the code (casts in particular), but I don't really see the advantage of using the SerializedATNView class. Does it really speed up deserialization? Do you have any evidence?

[jcking]

SerializedATNView avoids having to make yet another copy of the serialized ATN. This is already done once during static initialization (before main), and this would require it be done again resulting in 2 copies of the serialized ATN. Allocations are slow and avoiding them improves speed, especially when we really only need to subtract 2 for each element. The SerializedATNView basically just wraps the vector and subtracts 2 at each access, to keep code simple.

[mike-lischke]

Increase complexity to make things smaller and faster. Yeah, that's what C++ is known for 🤣

[mike-lischke]

@parrt OK, this one is also ready for merge.