- 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.

Implement Keys and Values as views in MapField

 Make FieldDescriptor.IsPacked work appropriately.

Remove our own version of MakeTag now that the main one is fixed.

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.

Tidying up

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

Fix groups handling in C#

"Tested" by regenerating code and observing that there are no changes.

Down integrate. Generate a package name suffix ".nano" for nano messages

Remove unused .pb.h #includes.

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.)

Change ReadTag/PeekTag behaviour to return 0 at EOF

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.

C# conformance tests

We should now have no conformance failures.

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

The tests are run from Travis in the same way as on other
platforms. Currently some expected failures - but only expected
in that they're what we got to start with. Will try to fix them in
other pull requests.

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

Document everything, and turn on errors if we fail to document anything in the future

Fix build warnings around unused variables

Various compilation fixes