(And likewise ignore the prefix in unpack.)

Fixes issue #1459.

(There are documentation changes and new fields in descriptor.proto that have resulted
in changes to the serialized descriptor, but no breaking changes for C#.)

(JSON parsing already does the right thing.)

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

Overview of changes:
- A new C#-specific command-line option, legacy_enum_values to revert to the old behavior
- When legacy_enum_values isn't specified, we strip the enum name as a prefix, and PascalCase the value name
- A new attribute within the C# code so that we can always tell the original in-proto name

Regenerating the C# code with legacy_enum_values leads to code which still compiles and works - but
there's more still to do.

(NCrunch noticed these.)

Introduce a new nuget package, Google.Protobuf.Tools, basically to contain protoc on multiple platforms.

I've moved both protoc.exe and the proto files out of Google.Protobuf.
The .proto files aren't a slam-dunk, but it feels like they belong with protoc as you'd *use* them with protoc.
It's not clear to me whether we really need both an x86 and x64 version of protoc.exe, as x86 would work on 64-bit Windows anyway. Discuss :)

This makes no externally visible behavioral changes. Internally and non-behaviorally:

- We use a field (compiler-generated) to store the JsonName to avoid recomputing it repeatedly
- The documentation for JsonName is updated to reflect the meaning better
- Readonly autoprops and expression-bodied properties used where possible

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

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.)

This was in an attempt to fix the wrapper handling corner case, but it's really fiddly.

This detects:
- An end-group tag with the wrong field number (doesn't match the start-group field)
- An end-group tag with no preceding start-group tag

Fixes issue #688.

This is a start to fixing issue #1212. It won't help for test protos,
conformance etc, but it will definitely be better than nothing, and
would have highlighted a change in descriptor.proto which broken C#
earlier.

Explicitly don't add any dependencies for "old" platforms, to avoid unnecessary
dependencies in those cases.

Fixes issue #1203.

(Generated code changes in next commit.)

Recently, descriptor.proto gained a GeneratedCodeInfo message, which means the generated code conflicts with our type.
Unfortunately this affects codegen as well, although this is a part of the public API which is very unlikely to affect hand-written code.

Generated code changes in next commit.

The usage of ICustomDiagnosticMessage here is non-essential - ToDiagnosticString
doesn't actually get called by ToString() in this case, due to JsonFormatter code. It was
intended to make it clearer that it *did* have a custom format... but then arguably I should
do the same for Value, Struct, Any etc.

Moving some of the code out of JsonFormatter and into Duration/Timestamp/FieldMask likewise
feels somewhat nice, somewhat nasty... basically there are JSON-specific bits of formatting, but
also domain-specific bits of computation. <sigh>

Thoughts welcome.

"valueField": null

is parsed appropriately, i.e. that it remembers that the field is set.