Switch to C++ streams for index saving and loading

[dokempf]

This PR introduces C++ streams instead of the C file handles used before. The intent here is to make the library more consistent w.r.t. its programming language usage and enable specific use cases that are hard to realize portably with C file handles (e.g. in-memory streams).

Some comments in response to @jlblancoc s comments in #156:

• Using <iosfwd> did not work out, as it only contains forward declarations of the stream classes, so we cannot use stream methods like read and write directly in the header. My compiler only produced warnings for this, but they are rather annoying. Instead, I had to include <istream> + <ostream>.
• The Doxygen annotations already state that we save in binary format. Nothing changed about that.

This fixes #156.

[jlblancoc]

Great, thanks! 👍