Part#2 (final): rewritting the code to pass the correct enum_sp_aggregate_type
to the sp_head constructor, so sp_head never changes its aggregation type
later on. The grammar has been simplified and defragmented.
This allowed to check aggregate specific instructions right after
a routine body has been scanned, by calling new LEX methods:
  sp_body_finalize_{procedure|function|trigger|event}()

Moving some C++ code from *.yy to a few new helper methods in LEX.