Fast-path "S" and "u" formats in cursor.get_key and cursor.get_value.

Michael, here are a couple of simple changes to inline "S" and "u" formats based on Sue's email on the LevelDB benchmark (strings are  obvious, and apparently "u" is used by LSM).

If you want another approach, or I missed a reason this isn't OK, or you want other formats included, I'm happy to own that work, just let me know.