v0.2.0

Breaking Changes

• tree::State::read() now returns an Arc containing the state, rather than a
  read guard. This change has no noticable impact on microbenchmarks, but yields
  more fair writing performance under heavy-read conditions -- something the
  current microbenchmarks don't test but in-development Commerce Benchmark for
  BonsaiDb unvieled.
• Buffer has been renamed to ArcBytes. This type has been extracted into its
  own crate, allowing it to be used in bonsaidb::core. The new crate is
  available here.
• Root::scan, Tree::scan, Tree::get_range, TransactionTree::scan, and
  TransactionTree::get_range now take types that implement
  RangeBounds<&[u8]>. BorrowByteRange is a trait that can be used to help
  borrow ranges of owned data.

Added

• nebari::tree::U64Range has been exposed. This type makes it easier to work
  with ranges of u64s.