Add hypothesis-protobuf library to the 3rd party doc.

unittest_proto3 had been changed in a very backward-incompatible
way which was never going to work with C# as it imports proto2 messages.

This is now a copy of the old file, but with a package name change for
compatibility with the remaining files in src/google/protobuf.

The other moves are for files that are only used by C#.

This will cause failures if shared protos change in an incompatible
way.

C#: Allow message parsing from an array slice

Update google/benchmark submodule to v1.2

* 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

When comparing message, require them to have the same descriptor.

This will cover someone subclassing the message, and also handles
something crazy like someone comparing to a raw NSObject.

If messages A and B have the same oneof case, which is a message
type, and we merge B into A, those sub-messages should be merged.

Fixes #3200.

Note that I haven't regenerated all the code, as some of the protos
have been changed, breaking generation.

Fixed some build issues on ppc64le architecture

update joda-time javadoc url

Note that the default behavior is still to throw an exception; you
need to opt into ignoring unknown fields.

Fixes #2838.

Suppress VS2017 compiler/linker warnings

* 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

Merge 3.4.x into master before cutting 3.5.x

To be clear that we set a new variable

Improves coverage of GPBCodedInputStream

Add brief documentation describing each warning suppression.

[v3.4.x branch] Fix iOS cc_library build for protobuf.

Fix iOS cc_library build for protobuf.

CodedInputStream::SetTotalBytesLimit description fix

fix implicit fallthrough in gcc 7

Integrate google internal changes for the up coming 3.5 release.

fixes issue #3750

Add @Deprecated annotation support for proto file.

Add the proto-lens Haskell library to the 3rd party doc.

protobuf_generate: create include path only for proto files