convert std::stringstream message use to one-liners

Summary:

We have a lot of code to convert to strings that uses stringstream as a builder:

std::stringstream ss;
ss << "foo" << x << std::endl;
std::string s = ss.str();

I'm pretty sure this is OK as a one-liner:

std::string s = (std::stringstream() << "foo" << x << std::endl).str();

I tested that it works on clang++ and g++ in a simple case. And I think it should work in general because str() returns a copy:

http://www.cplusplus.com/reference/sstream/stringstream/str/

The next question is whether the pattern could be used as an argument, and I think so given that the string is copied.

It would be even better if we could do this without explicitly including <sstream>. But there's nothing in Stan that's automatically brought in, so there's nowhere to put a typedef in stan::math that will automatically get picked up in every file.

Current Version:

v2.16.0

No files or tests are named. Start by searching the repository for the shown std::stringstream builder pattern and inspect each use before deciding whether the one-liner is safe; done means applicable conversions are made without breaking the existing build or tests.