Feature: add option to avoid nested messages to be under "Types" class.

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Consider the following .proto file:

message OutterMsg {
  message InnerMsg {}
  optional InnerMsg inner_msg = 1;
}

Currently, produced code looks like this:

class OutterMsg {
  class Types {
    class InnerMsg {...}
  }
  Types.InnerMsg InnerMsg;
}

Which means that to declare a builder for InnerMsg, one needs to type:

  OutterMsg.Types.InnerMsg msg;

That's rather long.

It's known that "Types" nested class is used to avoid clashing sub-message and 
Property names.

Another way to avoid such problem is to prefix sub-message names. For instance, 
a leading underscore char should suffice for 99% of the cases. Produced code 
would look like the following:

class OutterMsg {
  class _InnerMsg {...}
  _InnerMsg InnerMsg;
}

Declaration would look like:

  OutterMsg._InnerMsg msg;

A per-message (and perhaps a per-proto-file) option string 
csharp_nesting_prefix should work just fine. Proto file would look like the 
following:

message OutterMsg {
  option (csharp_nesting_prefix) = "_";

  message InnerMsg {}
  optional InnerMsg inner_msg = 1;
}

Original issue reported on code.google.com by igorga...@gmail.com on 21 Feb 2014 at 11:22