These statements pulled a bunch of symbols from the std namespace into
the global namespace. This commit removes all of them except for
std::string, which is a bit trickier to remove.

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

 Add unknown field support for csharp

* Fix php well known type conformance tests

* Properly generate code for test.proto

* Provide GPBMetadata files in c extensions for generated files to import.

* Remove unnecessary test

* Clean up code

* Add declaration for initOnce.

* Refactoring

* Fix php well known type conformance tests

* Properly generate code for test.proto

* Provide GPBMetadata files in c extensions for generated files to import.

* Remove unnecessary test

* Clean up code

* Add declaration for initOnce.

* Refactoring

* Reserve unknown in ruby

* Revert ruby tests. Wait for cpp impl for conformance test

* Add conformance test for preserving unknown

* Add unknown field conformance test to csharp failure list.

* Fix comments

* Fix comment

* Fix comments

* Fix typo

* Use stringsink_string directly

* Mark hd unused

* Remove unused encodeunknown_handlerfunc

This reverts commit 2bd55a9f.

* Fix js conformance tests.

* Remove old incorrect compatibility tests

http://python-future.org/compatible_idioms.html#file

Previously we only rejected the tag if the tag itself was 0, i.e.
field=0, type=varint. The type doesn't matter: field 0 is always
invalid.

This removes the last of the C# conformance failures.

- Better error message for unknown messageType.
- Remove unneeded if.

Both test_messages_proto3.proto & test_messages_proto2.proto define
message ForeignMessage {...} and enum ForeignEnum {...} but since
objc doesn't use the proto package in the naming, these end up
conflicting.

Adding the objc_class_prefix option to the proto3 file ensure the generated
objc types are all unique.

* Remove using std::{set,map}

* Add json encode/decode for php.

* Fix php conformance test on 32-bit machines.

* Fix conformance test for c extension.

* Fix comments