broken with recent changes' (attempt 2).
Adding Statement_core is better because:
- set_statement() code is shorter and you don't need to modify it when adding
new members to Statement_core
- a bit faster (you don't have virtual call and don't free_root() twice)
Do that short patch instead in hope that set_statement() will be sooner or
later removed entirely


sql/sql_class.cc:
  short patch for bug #2184 'Prepared statements in embedded library was 
  broken with recent changes'
sql/sql_class.h:
  short patch for bug #2184 'Prepared statements in embedded library 
  was broken with recent changes'