Simpify json tests

Fix C# JSON field ordering

Fix build failure on Windows when Unicode build is enabled.

Fix trivial bug in field orderings.

Use ' instead of " in the expected JSON, then replace it before asserting.

(Shows the benefit of unit testing even code "too simple to fail"...)

Well-known type operations for Timestamp and Duration

While I've provided operators, I haven't yet provided the method equivalents. It's not clear to me that
they're actually a good idea, while we're really targeting C# developers who definitely *can* use the user-defined operators.

Handle field names of "descriptor" and "types".

This is a follow up CL for e9abc404,
which breaks build when UNICODE macro is defined.

protoc has explicitly called MBCS version of APIs / funcsions
rather than UTF-16 (wchar_t) version of them regardless of
UNICODE macro definition (and it indeed works as expected).
Hence it makes sense to call GetModuleFileNameA explicitly.

Expose GetUmbrellaClassName in csharp_names.h

Merge C# proto3 from csharp-experimental to master

Throw on null for string/bytes single fields

Rename ThrowHelper to Preconditions and make it public - we'll want to use it from the generated code soon.

Additionally, change it to return the value passed, and make it generic with a class constraint.
A separate method doesn't have the class constraint, for more unusual scenarios.

Remove a few hangers-on from frozenness.

Remove the C# Freeze API

Expose IsClientStreaming and IsServerStreaming in MethodDescriptor

Move the definition of GOOGLE_FALLTHROUGH_INTENDED to port.h

On x64 MSVC platform, macro PROTOBUF_LITTLE_ENDIAN won't be set

First attempt at using profile 259 for Google.Protobuf.

- Fix nupec paths
- Remove an obsolete part of the JSON build
- Add documentation and tests to reflection extension methods, and improve implementations

Update minimal ios and osx version for deployment.

Remove two unused functions.