4.4.0

New features

• Implement Value and Key for the NonZero integer types, NonZeroU8 to NonZeroU128 and
  NonZeroI8 to NonZeroI128.
• Add Value::NICHE, an optional byte string that a type never encodes to. When a type declares
  one, Option of that type encodes None as it, in place of a tag byte. Adding a niche to a type
  changes the encoding of Option of it, and its type name with it, so the two are not equivalent
  types: a table that stores Option of the type from before it declared the niche fails to open
  with TableTypeMismatch instead of being misread, and can be read through a copy of the type
  that declares no niche.
• Stabilize ReadableTable::lower_bound() and ReadableTable::upper_bound(), which return a read-only
  Cursor, modeled on the standard library's BTreeMap cursors.
• Stabilize Table::lower_bound_mut() and Table::upper_bound_mut(), which return a CursorMut
  modeled on the standard library's BTreeMap cursors. Inserting sorted data through
  insert_before() can be around 3x faster than insert().

Bug fixes

• Fix a bug where check_integrity() could report that it repaired corruption, after a transaction
  that grew the file size was aborted.
• Fix Database::create() repairing the database it had just created, which failed with
  RepairAborted under a repair callback that aborts.
• Fix unbounded memory growth in the page cache under write workloads, when the cache never
  reached its configured size limit.
• Fix Durability::None commits being lost on a clean close, after a panic was caught inside
  an earlier write transaction.
• Fix compact() leaving free regions at the end of the file, when the compaction emptied more
  of them than it made commits.
• Fix compact() waiting forever when the caller holds a write transaction. It fails with
  CompactionError::TransactionInProgress, as it does for a read transaction.