do not incorrectly mark the column as fixed-length.  (Bug #28138)

ibuf_entry_build(): Instead of prefix_len, pass fixed_len to
dtype_new_store_for_order_and_null_size().  Add debug assertions.

btr_index_rec_validate(): Correct a comment about prefix indexes.

rec_get_converted_size_new(), rec_convert_dtuple_to_rec_new(): Add
debug assertions and comments.

dict_col_type_assert_equal(): New debug function.