Fixed backpatching of forward jumps.
Implemented LOOP, WHILE, REPEAT (temporarily known as SPREPEAT).

Known bug: Expression evaluation still not quite ok (e.g. "x > 0"),
which is why IF and CASE is not yet implemented.


sql/Makefile.am:
  Added new sp.h/sp.cc file.
sql/item.h:
  New deferred result_type() method in Item_splocal.
sql/lex.h:
  Temporary fix until REPEAT conflict is solved. Use SPREPEAT for now.
sql/sp_head.cc:
  Moved create/find/drop functions to sp.cc.
  Fixed the backpatch stuff.
  (Also removed some dead code and updated comments.)
sql/sp_head.h:
  Moved create/find/drop declarations to sp.h.
  Fixed the backpatch stuff.
sql/sp_pcontext.h:
  New method: last_label().
sql/sql_parse.cc:
  Include sp.h.
sql/sql_yacc.yy:
  Fixed backpatching of forward jumps.
  Implemented LOOP, WHILE, and REPEAT. (Note: SPREPEAT for now.)