Add methods that allow to insert/fetch a stream of bytes in arbitrary format

[slvrtrn]

See the discussion here: #152 (comment)

For raw insert, the main use case is to insert the data from files. Similarly, with raw query, the data can be saved into a file.

Possible implementations:

• Query::fetch_raw (returning just RawCursor, so it and it's next method will be public)
• Insert::write_raw?