Add hyper:precision/1

Provide a getter for the precision of a HyperLogLog filter.
Merge pull request #5 from martinrehfeld/master

Add hyper:precision/1
Support mixed-precision union

... for the hyper_binary default backend only, sorry ;-)
Demonstrate error rate of reduced-precision HLLs
Merge pull request #6 from martinrehfeld/folding

Support mixed-precision union