Generated code changes from previous commit

Note that now we need a proto3 version of addressbook.proto. This may affect other platforms, and could do with an overhaul to follow proto3 conventions anyway (e.g. repeated field names). Will need to think about that carefully before merging into master. Raised issue #565 for this.

Prod code works, but some tests are broken. Obviously those need fixing, then more tests,
and review benchmarks.

No specific test case - if the generated code compiles, the issue is fixed :)

(Primarily this is starting the hash code of messages at a non-zero value...)

Fixes issue #523.

Fixes issue #527.

- Make some members internal
- Remove a lot of FrameworkPortability that isn't required
- Start adding documentation comments
- Remove some more group-based members
- Not passing in "the last tag read" into Read*Array, g

Remove ICodedInputStream and ICodedOutputStream, and rewrite CodedInputStream and CodedOutputStream to be specific to the binary format. If we want to support text-based formats, that can be a whole different serialization mechanism.

This makes repeated fields really awkward at the moment - but when we reimplement RepeatedField<T> to be backed by an array, we can cast the array directly...

This commit includes changes to the C#-specific protos, and rebuilt versions of the "stock" protos.
The stock protos have been locally updated to have a specific C# namespace, but this is expected to change soon, so hasn't been committed.
Four areas are currently not tested:
1) Serialization - we may restore this at some point, possibly optionally.
2) Services - currently nothing is generated for this; will need to see how it interacts with GRPC
3) Fields beginning with _{digit} - see https://github.com/google/protobuf/issues/308
4) Fields with names which conflict with the declaring type in nasty ways - see https://github.com/google/protobuf/issues/309