Post-review fixes according to Monty's review.


sql/item.h:
  Unite all code that stores and restores the state of a name resolution context
  into a class to represent the state, and methods to save/restore that
  state.
sql/mysql_priv.h:
  Reorder parameters so that length is after the name of a field,
  and database is before table name.
sql/sql_acl.cc:
  Reorder parameters so that length is after the name of a field,
  and database is before table name.
sql/sql_base.cc:
  * Reorder parameters so that length is after the name of a field,
    and database is before table name.
  * Added new method - Field_iterator_table_ref::get_natural_column_ref
    to avoid unnecessary code when it is knwon that no new columns will
    be created when accessing natural join columns.
sql/sql_insert.cc:
  Unite all code that stores and restores the state of a name resolution context
  into a class to represent the state, and methods to save/restore that
  state.
sql/sql_lex.cc:
  Removed obsolete comment.
sql/sql_lex.h:
  Return error from push_contex() if there is no memory.
sql/sql_list.h:
  Extended base_list_iterator, List_iterator, and List_iterator_fast with an
  empty constructor, and init() methods, so that one doesn't have to construct
  a new iterator object every time one needs to iterate over a new list.
sql/sql_parse.cc:
  Moved common functionality from the parser into one function, and renamed
  the function to better reflect what it does.
sql/sql_yacc.yy:
  Moved common functionality from the parser into one function, and renamed
  the function to better reflect what it does.
sql/table.cc:
  * Extended base_list_iterator, List_iterator, and List_iterator_fast with an
    empty constructor, and init() methods, so that one doesn't have to construct
    a new iterator object every time one needs to iterate over a new list.
  * Added new method Field_iterator_table_ref::get_natural_column_ref to be
    used in cases when it is known for sure that no new columns should be
    created.
sql/table.h:
  - column_ref_it no longer allocated for each new list of columns
  - new method get_natural_join_column for faster/simpler access
    to natural join columns.