Add preserve UnknownFields
Compare floating point values bitwise
Add auto-generated header to C# generated files

This is the manual code part of the Google.Protobuf library, and tests.
Some tests will fail until codegen is changed and rerun.

This is the only call to TypeExtensions.IsValueType, so we can remove
that method, making the whole type conditionally compiled out for .NET 3.5

Previously we were incorrectly packing wrapper types.
This also refactors FieldCodec a bit as well, using more C# 6-ness.

(The method was last used a very long time ago, if ever.)

This includes all the well-known types except Any.
Some aspects are likely to require further work when the details of the JSON parsing expectations are hammered out in more detail. Some of these have "ignored" tests already.

Note that the choice *not* to use Json.NET was made for two reasons:
- Going from 0 dependencies to 1 dependency is a big hit, and there's not much benefit here
- Json.NET parses more leniently than we'd want; accommodating that would be nearly as much work as writing the tokenizer
This only really affects the JsonTokenizer, which could be replaced by Json.NET. The JsonParser code would be about the same length with Json.NET... but I wouldn't be as confident in it.

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

- We do still generate the message types, as otherwise reflection breaks, even though it doesn't actually use those types.
- JSON handling hasn't been implemented yet

... and some implementation changes to go with them.

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.

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

- Change the default message hash code to 1 to be consistent with other code
- Change the empty list/map hash code to 0 as "empty map" is equivalent to "no map"
- Removed map fields from unittest_proto3.proto
- Created map_unittest_proto3.proto which is like map_unittest.proto but proto3-only
- Fixed factory methods in FieldCodec highlighted by using all field types :)
- Added tests for map serialization:
  - Extra fields within entries
  - Entries with value then key
  - Non-contiguous entries for the same map
  - Multiple entries for the same key

Changes to generated code coming in next commit

More tests required. Generated code in next commit.