We still need the JSON representation, which relies on something like a DescriptorPool to fetch message types from based on the type URL. That will come a bit later.
(The DescriptorPool comment in this commit is just a note which will prove useful if we use DescriptorPool itself.)

We now do this in protoc instead of the generation simpler.

Benefits:
- Generation script is simpler
- Detection is simpler as we now only need to care about one filename
- The embedded descriptor knows itself as "google/protobuf/descriptor.proto" avoiding dependency issues

This PR also makes the "invalid dependency" exception clearer in terms of expected and actual dependencies.

We now do this in protoc instead of the generation simpler.

Benefits:
- Generation script is simpler
- Detection is simpler as we now only need to care about one filename
- The embedded descriptor knows itself as "google/protobuf/descriptor.proto" avoiding dependency issues

This PR also makes the "invalid dependency" exception clearer in terms of expected and actual dependencies.

With this in place, generating APIs on github.com/google/googleapis works - previously annotations.proto failed.
Currently there's no access to the annotations (stored as extensions) but we could potentially expose those at a later date.

- Removed a TODO without change in DescriptorPool.LookupSymbol - the TODOs were around performance, and this is only used during descriptor initialization
- Make the CodedInputStream limits read-only, adding a static factory method for the rare cases when this is useful
- Extracted IDeepCloneable into its own file.

This is a bit of a grotty hack, as we need to sort of fake proto2 field presence, but with only a proto3 version of the descriptor messages (a bit like oneof detection).
Should be okay, but will need to be careful of this if we ever implement proto2.

Visual Studio is happy with it, but Mono isn't :(

Now the generated code doesn't need to check for end group tags, as it will skip whole groups at a time.
Currently it will ignore extraneous end group tags, which may or may not be a good thing.
Renamed ConsumeLastField to SkipLastField as it felt more natural.
Removed WireFormat.IsEndGroupTag as it's no longer useful.

This mostly fixes issue 688.

(Generated code changes coming in next commit.)

We don't need to expose the InvalidProtocolBufferException factory method now that the generated code doesn't throw the exception.

Change ReadTag and PeekTag to just use 0 as a return value for "end of stream", rather than using an awkward out parameter.
This simplifies quite a lot of code.

Generated code in next commit.

We should now have no conformance failures.

This is expected to be the cause of the conformance test failures.
Generated code in next commit.

Completely untested so far - easier to get started in VS and then transfer to Linux for tweaking...

Fixes issue #552. (And yay, it looks like our build profile supports this...)

This seems remarkably little code, but it appears to work. I can add tests for invalid structs at some point, once the general approach is approved.