Merge 'Implement SQLite balancing algorithm' from Pere Diaz Bou

Beep boop.
What happened you ask? I removed the dumb balancing algorithm I
implemented in favor of SQLite's implementation based on B*Tree[1] where
a page is 2/3 full instead of 1/2. It also tries to balance a page by
taking a maximum 3 pages and distributing cells evenly between them.
I've made some changes that are somewhat related:
* Moved most operations on pages out of BTreeCursor because those
operations are based on a page, not on a cursor, and it makes it easier
to test.
* Fixed `write_u16` and `read_u16` cases that didn't need a implicit
offset calculation. Added: `write_u16_no_offset` and
`read_u16_no_offset` to counter this.
* Added some tests with fuzz testing too.
* Fixed some important actions like: `compute_free_space`,
`defragment_page` and `drop_cell`.
[1] https://dl.acm.org/doi/10.1145/356770.356776

Closes #968

core/lib.rs

@@ -227,8 +227,8 @@ pub fn maybe_init_database_file(file: &Rc<dyn File>, io: &Arc<dyn IO>) -> Result
             btree_init_page(
                 &page1,
                 storage::sqlite3_ondisk::PageType::TableLeaf,
-                &db_header,
                 DATABASE_HEADER_SIZE,
+                db_header.page_size - db_header.reserved_space as u16,
             );
 
             let contents = page1.get().contents.as_mut().unwrap();