sql. MsgPack subtype

 * Introduce MsgPack subtype for Mem. A BLOB with MsgPack subtype is
   assumed to contain MsgPack-encoded data.
 * OP_Column sets MsgPack subtype when reading back arrays, maps or
   extensions from a row field.
 * OP_MakeRecord emits fields with MsgPack subtype verbatim (ordinary
   BLOBs are prefixed with MsgPack BIN header.)
 * Extend OP_Blob to support setting BLOB's subtype.

sql. Add functions to encode spaceid,iid in a PageNo