v2.0.0

• Replace all optional arguments with an *Options struct in constructors.
• All subclasses now have the final tag to assist compiler devirtualization.
• PerByte is now an interface class that has serial and parallel subclasses to enable switching at run-time.
• Removed alternative constructors for std::string paths or char buffers, for simplicity.
• IstreamReader and OstreamWriter now hold an unique pointer to std::stream to clarify ownership.
• Support modification of the internal buffer sizes in zlib and cstdio, in their respective reader/writer classes.