Note that the compatibility tests have had to cahnge as well, to
cope with internal changes. (The test project has access to
internals in the main project.)

Fixes #3209.

- Add a TryUnpack method which doesn't throw if the type is wrong
- Make GetTypeName public for easier determination of the message type

Fixes #3294.

This has one important packaging change: the netstandard version now
depends (implicitly) on netstandard1.6.1 rather than on individual
packages. This is the preferred style of dependency, and shouldn't
affect any users - see http://stackoverflow.com/questions/42946951
for details.

The tests are still NUnit, but NUnit doesn't support "dotnet test"
yet; the test project is now an executable using NUnitLite. (When
NUnit supports dotnet test, we can adapt to it.)

Note that the project will now only work in Visual Studio 2017 (and
Visual Studio Code, and from the command line with the .NET Core
1.0.0 SDK); Visual Studio 2015 does *not* support this project file
format.

There might be duplicated enum values when allow_alias is true. Add PreferredAlias into OriginalNameAttribute to remove the duplication (#2727)

NUnit 3.4.0 —> 3.6.0
dotnet-test-nunit 3.4.0-alpha-2 —> 3.4.0-beta-3

* Changing DOTNET35 framework symbols in preprocessor directives to the default built-in value of NET35.
* Adding extension method StreamExtension.CopyTo for .NET 3.5 because it didn’t exist until .NET 4, and adding associated unit tests.

Fixes #2669.

This consists of:
- Changing the codegen for the fixed set of options protos, to parse unknown fields instead of skipping them
- Add a new CustomOptions type in the C# support library
- Expose CustomOptions properties from the immutable proto wrappers in the support library

Only single-value options are currently supported, and fetching options values requires getting the type right
and knowing the field number. Both of these can be addressed at a later time.

Fixes #2143, at least as a first pass.

Fixes #2088.

We now have separate tests for netcoreapp and net45 to test the two branches here.
(netstandard10 doesn't have MemoryStream.GetBuffer)

Although most of this library doesn't have any async functionality,
this feels like a natural place to locally add it.

Fixes #2511.

* Factored Conformance test messages into shared test schema.

* Updated benchmarks to use new proto3 message locations.

* Fixed include path.

* Conformance: fixed include of Python test messages.

* Make maven in Rakefile use --batch-mode.

* Revert changes to benchmarks.

On second thought I think a separate schema for
CPU benchmarking makes sense.

* Try regenerating C# protos for new test protos.

* Removed benchmark messages from test proto.

* Added Jon Skeet's fixes for C#.

* Removed duplicate/old test messages C# file.

* C# fixes for test schema move.

* Fixed C# to use the correct TestAllTypes message.

* Fixes for Objective C test schema move.

* Added missing EXTRA_DIST file.

This change fixes the following Chromium presubmit error:

  third_party/protobuf/csharp/src/Google.Protobuf.Test/project.json could
  not be parsed: Expecting property name: line 25 column 3 (char 482)

This affects cases with leading capital letters.

This breaks compatibility with previous C# releases, but
fixes compatibility with other implementations.

See #2278 for details.

(This still doesn't fix the conformance tests, but at least
we're now consistent with the C++ code.)

(This still doesn't fix the conformance tests, but at least
we're now consistent with the C++ code.)

This also updates the version number to 3.0.0-beta4

We now just perform the optimization within AddRange itself.

This is a breaking change in terms of "drop in the DLL", but is
source compatible, which should be fine.

(Also fix a few more C# 6-isms.)

This fixes issue #1730.

Move to dotnet cli for building, and .NET Core (netstandard1.0) as target platform

This also updates the version number to 3.0.0-beta4

* Improve exception throwing implementation in collections

* Implement RepeatedField.AddRange.

This fixes issue #1730.

* Optimize AddRange for sequences implementing ICollection

(Also fix a few more C# 6-isms.)

* Remove the overload for Add(RepeatedField<T>)

We now just perform the optimization within AddRange itself.

This is a breaking change in terms of "drop in the DLL", but is
source compatible, which should be fine.

This is the only call to TypeExtensions.IsValueType, so we can remove
that method, making the whole type conditionally compiled out for .NET 3.5

* Adding condition compiler symbol to support .NET 3.5

This isn't useful to most users, but can be handy in advanced use cases,
as requested in #1465.

Also added a standalone formatter test, for confidence.
Have validated that undoing the change in 835fb947 breaks the tests
(i.e. we are still testing that the change is required).

(And likewise ignore the prefix in unpack.)

Fixes issue #1459.

JSON tests fail, as we're not using OriginalNameAttribute yet.

(NCrunch noticed these.)

Conflicts:
	csharp/src/Google.Protobuf/JsonFormatter.cs