(It doesn't yet, but will in the next commits...)

Most changes are:

- Introducing new helpers of SupportsPresenceApi and RequiresPresenceBit. This allows calling code to be a lot clearer about what it's interested in.
- Changing most previous IsProto2 calls to use one of the two new helper methods
- Avoiding treating synthetic oneofs as regular ones
- Some slight refactoring in csharp_primitive_field to avoid code duplication
- Comments explaining what we want when, so the next maintainer doesn't need to do the detective work I did!

This change deliberately doesn't modify the API surface of any
existing code. The only change to previously-generated C# should be
making presence bits more efficient in proto2.

Once proto3 optional fields are supported, we can consider further
changes to make the proto2 and proto3 generated API surface more
consistent (e.g. adding presence API for message fields and oneofs).

…ional.proto

The changes in the existing proto2 code are solely around presence bits. The new generator allocated presence bits more efficiently. (Previously bits were sometimes allocated but never used.)

(This isn't as exhaustive as it might be, but the behavior is basically the same as proto2 optional fields.)

This is more involved than might be expected because the synthetic oneofs don't generate the properties we would usually expect to see.