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'll see what I've missed when CI fails...

- Move types into Google.Protobuf.Reflection
- Change codegen to reflect that in generated types

Generated code changes coming in part 2

Everything should now be consistent - the only difference between files is the year (aside from generated files).

Fixes issue #531.

This is mostly just making things internal instead of public, removing and reordering a bunch of code in CodedInputStream/CodedOutputStream, and generally tidying up.

We need to remove it from the generator too; I'll raise a github issue for that.