MinidumpCallback passes a wstring parameter

This means that it is very tempting for the implementor of MinidumpCallback to call the .c_str() function, which in many std::wstring implementations results in a heap allocation (to make space for the null terminator). I think the airbag API should pass a |const wchar_t*| instead to help steer consumers away from making the mistake of using the heap after a crash.

Will roll in to patch for #81.

Checked in with #81, revision [79].