- HDRHistogram-compatible serialization/deserialization (#28, #43)
 - Restrict supported counter types (#49, #29)
 - Reduced memory footprint (#12)
 - More extensive test suite (#8, #18, #24, #25)
 - Better handling of errors in edge-cases (#16, #22, #34, #50)
 - Removed criterion (#46, #23)
 - Various performance improvements (#15, #26, #48)

In terms of backwards-incompatible changes, the major ones are:

 - Only u8-u64 counters are now supported. Support for iXX and fXX
   variants was removed as they end up with questionable logic around
   negative and non-integer numbers. See #29 for details.
 - More functions now return `Result` where they would previously just
   panic or encounter undefined behavior. This has changed the APIs in a
   couple of places, though adapting to it should be straightforward.