accessors returning pointers with macros that preserve const qualifiers.
In UNIV_DEBUG builds, retain the accessors and cast away constness there.
dfield_get_type(), dfield_get_data(), dtuple_get_nth_field(),
dict_table_get_nth_col(), dict_table_get_sys_col(): Implement as macro
unless #ifdef UNIV_DEBUG.
rec_get_nth_field(): Replace with rec_get_nth_field_offs() that does not
do pointer arithmetics. Implement rec_get_nth_field() as a macro.
and modify some functions to return const pointers. Add const qualifiers
to local variable declarations or casts to remove the const qualifier
in those places where write access is needed.
