v6.0.0

This release has a couple of backwards-incompatible changes:

• the old len() is now distinct_values()
• the new len() is the old count() (which is deprecated)
• IterationValue::value became value_iterated_to

Some other API changes:

• iterator values gained quantile_iterated_to()
• Histogram gained is_empty()

Behind the scenes:

• #67 and #68 landed a number of fixes to iterators such that the
  produced values are more correct and sensible.
• errors were moved into their own module.