protoparse: match the way protoc populates aggregate value in uninterpreted options

[jhump]

This dumps the original source input for the aggregate as its string value, but we strip formatting/whitespace/comments and just put a single space between each token (also omit enclosing braces). This is what protoc does.

This patch was ported over from https://github.com/jhump/protocompile/pull/6.

Fixes #274.

---

To confirm the output of protoc, beyond inspecting the implementation code linked above, I wrote a simple C++ program that calls Parser::Parse but does not do any linking or options interpretation, so I could view how the uninterpreted options fields in the descriptor protos were populated.

Here's that test program, which reads proto source from stdin and spits out the resulting descriptor proto (text format) to stdout:

// file: just-parse.cc
#include <iostream>
#include <string>
#include <unistd.h>
#include <google/protobuf/io/zero_copy_stream_impl.h>
#include <google/protobuf/io/tokenizer.h>
#include <google/protobuf/compiler/parser.h>

class PrintingErrorCollector : public google::protobuf::io::ErrorCollector {
 public:
  PrintingErrorCollector() {}

  void AddError(int line, google::protobuf::io::ColumnNumber column, const std::string& message) {
    std::cerr << "ERROR: <input>" << ":" << line << ":" << column << ": " << message << "\n";
  }
  void AddWarning(int line, google::protobuf::io::ColumnNumber column, const std::string& message) {
    std::cout << "WARN: <input>" << ":" << line << ":" << column << ": " << message << "\n";
  }
};

int main(int argc, char *argv[]) {
  google::protobuf::io::FileInputStream in(STDIN_FILENO);
  google::protobuf::FileDescriptorProto fdp;
  PrintingErrorCollector coll;
  google::protobuf::io::Tokenizer tok(&in, &coll);
  google::protobuf::compiler::Parser parser;
  parser.RecordErrorsTo(&coll);
  parser.SetRequireSyntaxIdentifier(false);
  if (! parser.Parse(&tok, &fdp)) {
    std::cerr << "parse failed\n";
    return 1;
  }
  fdp.clear_source_code_info();
  fdp.PrintDebugString();
  std::cout << "\n";
  return 0;
}

In order to build this on my Macbook Pro, I had to build/install the protobuf C++ libraries/runtime and then run the following:

clang++ -std=c++11 just-parse.cc -o just-parse `pkg-config --cflags --libs protobuf`