Implement Freeze in C# protos

The solution as a whole doesn't build yet - we probably want to remove
ProtoDump and ProtoMunge entirely, and ProtoBench should use Jan's new
benchmarks for parity with Java.

The version of NUnit on my machine, packaged with Mono 3.12.1, is
only NUnit 2.4.2, which is extremely old - it still requires an explicit
[TestFixture] attribute on test fixtures. I've added one just for ByteStringTest
for the moment so that we can see some tests passing in Travis, but as part of
a separate PR we should work on making sure we're using a recent NUnit version.
(It may already be doing so, but we can check that once it's working and merged.)

It seems too much code relies on the broken behaviour. See issue #493.
Instead, we reimplement MakeTag just for C#, temporarily.

Fixes issue #523.

Integrate changes from upstream/master

Implement Clone.

1) New line at end of file
2) Make IMessage<T> itself extend IEquatable<T> and IDeepCloneable<T>

Update upb to fix two bugs in the Ruby library.

Fixes issue #527.

Fixes:
  https://github.com/google/protobuf/issues/502
  https://github.com/google/protobuf/issues/425

Add nonnull/nullable/null_resettable markup to ObjC library.

Proto3 experimental C# fork

- 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 OUTS constants in BUILD

Induce those from the input proto file names instead to reduce the
lenght of the BUILD file.

Change-Id: I1dda57dedac547f4bd0cc3a7d08fb611d72acd13

Update BUILD for the recent json util change.

Change-Id: I0b5a2c017f5412aaca3a47815671753cdef63165

using ::isnan to avoid conflicts with std::isnan for C++11 build.

Change-Id: I057b44eff04c3b0062a4fd4307ce6e4ca1eb952b

Change-Id: I74e2d5958b5e7b15d10e5a6468bca917323b9739

Add bazel support for the project

Change-Id: I62640af99e3c2650a29dc5468ce3deb7b6c7efdf

Change-Id: I5a299d969ff96d7d2f80aadc7e8987d461d24b8f

Change-Id: Idecb2e3a5169f7b40d832242c9c10b0eb4325654

Down-integrate from internal code base.

We'll probably want a lot of the code from the serialization project when we do JSON, but enough of it will change that it's not worth keeping in a broken state for now.