This PR addresses four missing features (plus two not mentioned there) listed in #735.

• Adds readv, writev, readvSync and writevSync to Volume.
• Adds FileHandle#readv and FileHandle#writev.

As a bonus, I modified the existing read implementation:

• modified the signature of read so that position can receive null, which better aligns to the readv signature. In fact, passing null was already working expect that TypeScript complained about that.
• passing -1 to position is now equivalent to passing null for better compatibility with Node.
• wrote missing tests.
• implemented range check for the “read more than buffer space” case.

I also modified the existing write implementation:

• passing -1 to position is now equivalent to passing null for better compatibility with Node.