Release 7.0.0

Release 6.3.4

Release 6.3.3

Release 6.0.0

Release 5.0.0

Lots of changes, mostly from @marshallpierce

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

A struct is now yielded for iteration values, rather than a hard-to-read

tuple (HdrHistogram#5). Furthermore, u64 is used for aggregated counts, even if the
underlying count type T is smaller than u64 (HdrHistogram#3). Thanks to
@marshallpierce for putting together most of this release!