2.0.0-beta0

Major file format change

2.0.0 uses a new file format that optimizes len() to be constant time. This means that it is not
backwards compatible with 1.x. To upgrade, consider using a pattern like that in
upgrade_v1_to_v2 test.

Other changes

• check_integrity() now returns a DatabaseError instead of StorageError
• Refactor table metadata methods into a new ReadableTableMetadata trait
• Rename RedbKey to Key
• Rename RedbValue to Value
• Remove lifetimes from read-only tables
• Remove lifetime from WriteTransaction and ReadTransaction
• Remove drain() and drain_filter() from Table
• impl Clone for Range
• Add len() and is_empty() to MultimapValue
• Add retain() and retain_in() to Table
• Add extract_if() and extract_from_if() to Table
• Add range() returning a Range with the 'static lifetime to read-only tables
• Add get() returning a range with the 'static lifetime to read-only multimap tables
• Add close() method to ReadTransaction