Codechange: [core] Implement SmallVector using std::vector

The public and protected interface to SmallVector are unchanged
SmallVector now requires that items be default constructible
This isn't an issue since some contained items were previously created
uninitialized.

Temporary default constructors are added to the following structs
- SmallPair
- SmallStackItem
- GRFPresence

Where vector<bool> is required, transition immediately to std::vector
to avoid returning proxy object references.