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.

This fixes issue #679 and issue #1282.
(The .gitignore change is just around ncrunch; I can put it in a separate PR if you really want.)

- Removed a TODO without change in DescriptorPool.LookupSymbol - the TODOs were around performance, and this is only used during descriptor initialization
- Make the CodedInputStream limits read-only, adding a static factory method for the rare cases when this is useful
- Extracted IDeepCloneable into its own file.

Change ReadTag and PeekTag to just use 0 as a return value for "end of stream", rather than using an awkward out parameter.
This simplifies quite a lot of code.

Generated code in next commit.

We'll see what I've missed when CI fails...

- Added new line at the end of SampleEnum
- Moved GeneratedMessageTest.GetSampleMessage to a new class, SampleMessages, and renamed it to CreateFullTestAllTypes.

Everything should now be consistent - the only difference between files is the year (aside from generated files).

Fixes issue #531.

This is mostly just making things internal instead of public, removing and reordering a bunch of code in CodedInputStream/CodedOutputStream, and generally tidying up.

- Make some members internal
- Remove a lot of FrameworkPortability that isn't required
- Start adding documentation comments
- Remove some more group-based members
- Not passing in "the last tag read" into Read*Array, g

Remove ICodedInputStream and ICodedOutputStream, and rewrite CodedInputStream and CodedOutputStream to be specific to the binary format. If we want to support text-based formats, that can be a whole different serialization mechanism.

This makes repeated fields really awkward at the moment - but when we reimplement RepeatedField<T> to be backed by an array, we can cast the array directly...

This includes the NUnit test adapter which allows NUnit tests to be run under VS without any extra plugins.
Unfortunate the compatibility tests using the abstract test fixture class show up as "external" tests, and aren't well presented - but they do run.

This includes fetching the VS unit test runner package, so that tests can be run from Visual Studio's Test Explorer.