This is effectively reimplementing List<T>, but with a few advantages:
- We know that an empty repeated field is common, so don't allocate an array until we need to
- With direct access to the array, we can easily convert enum values to int without boxing
- We can relax the restrictions over what happens if the repeated field is modified while iterating, avoiding so much checking

This is somewhat risky, in that reimplementing a building block like this is *always* risky, but hey...
(The performance benefits are significant...)