Fix up warnings from Xcode 9.1

Here's the pull request that made it happen: https://github.com/google/protobuf/pull/2951

Fixes 32bit tests.

Allow one to omit building libprotoc and protoc binaries

Travis cleanups

Reduce size of GPBDictionary by getting rid of class creation methods

Simplify getter/setter method implementations

Remove unreferenced 'GPBMessageSignatureProtocol' class.

Looking at tarvis history for master, jruby doesn't look like it has passed
in a really long time.  The other two tests have been failing for a few weeks.
Since travis has limited osx hosts, seems better to just remove things we
aren't expecting to pass.

Remove unnecessary retain/releases for GPBFieldDescriptors when
creating implementations with blocks. The descriptors do not need
to be memory managed because they are essentially global constructs
that are guaranteed to be around the lifetime of the app.

Remove unreferenced 'GPBMessageSignatureProtocol' class that is just taking up
space in the Objective C runtime information by hanging the protocol it needs to
declare off of GPBRootObject instead.

Small binary size reduction, but more importantly a removal of a class that
appears to be unused when statically analyzed.

Fix a typo in WKT's test suite

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.

Hi!
Sorry for bothering with such a minor thing, but I just noticed and fixed one annoying typo in WKT's test suite:
`"accpets" -> "accepts"`

Thanks!

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