Check for reserved words being used

[adamconnelly]

We should be able to add a check for various reserved words being used or other problems that will end up causing C# compilation errors. For example the following enum will cause a compilation error because it uses the reserved word class as its name:

enum class {
  Class1,
  Class2
}

In that particular situation we've got at least the following options:

1. Just don't allow it, and output an error message.
2. Rename class to @class in the generated code.
3. Rename to @class, and output a warning that we're having to automatically rename it.

Options 2 and 3 are probably the best option to support cross-platform thrift files where a name might use a reserved word in one language but not another, but obviously the first option is the simplest.

There may also be other situations where renaming like that isn't an option.