Fix build errors due to indexers

[jasongin]

Fixes: #270
Fixes: #271

• Exclude indexers from module exports for now, with a warning.
• Fix a couple things which are not really needed as long as indexers aren't exported by the module generator, but might be helpful in the future:
  • Handle indexer properties when converting expression trees to C# code.
  • Handle multiple indexers on one class when converting type symbols to types.

JavaScript doesn't support C# style indexers. (Technically it could with proxies, but I don't think we want to make every .NET object a proxy.) We could do something else like generate get/set methods for the indexer, but that would be more work. For now it's a good improvement that at least indexers won't break the build.