btr_root_raise_and_insert(): Because btr_page_set_level() must not
change level from non-zero to zero on compressed pages, invoke
btr_page_set_level() with page_zip==NULL and compress the entire
root page after creating it from the scratch.

btr_attach_half_pages(): Simplify the computation of lower_page_zip
and upper_page_zip.  Invoke btr_node_ptr_set_child_page_no() with
the correct page_zip.

page0page.h: Add __attribute__((const)) to functions testing for
infimum or supremum.

page_zip_dir_delete(): Note that the third parameter may be NULL.
Correct offset errors.

page_zip_available(): Use n_heap instead of n_recs.

page_zip_dir_find(), page_zip_dir_find_free(): Fix off-by-one error.

page_zip_fields_encode(), page_zip_fields_decode(): Encode and decode
index->n_nullable for non-leaf pages.

page_zip_apply_log(): Write REC_NEW_HEAP_NO before calling
rec_offs_make_valid().

page_zip_write_node_ptr(): Correct off-by-one error.

page_cur_search_with_match(): Make use of page_is_leaf().

page_dir_add_slots(): Replaced with page_dir_add_slot().  Use memmove().