Npgsql: Support reading/writing Span to/from bytea

[roji]

With the new Span introduced in C# 7.2, we should support it for PostgreSQL bytea.

[roji]

Some notes:

• Wait for 2.1, because right now there are no APIs for reading/writing Spans (e.g. in Stream)
• Possibly add an optimization for bytea for reading a ReadOnlySpan which points into the Npgsql buffer, in cases where the entire value fits in the buffer. That's zero-copy bytea reading, pretty cool.
• Unrelated to bytea - remove the tempCharBuffer used for skipping, allocate on the stack instead.