Support resetting histogram?

[brandonchinn178]

Currently, counter and gauge can be reset with set 0, but histogram doesn't have that functionality. I'm considering defining metrics as global variables, and it would be nice to reset metrics if necessary in tests

[brandonchinn178]

Prototype: 8c53715

[wraithm]

Opened #61 to add Histogram.reset.

It follows the approach from your prototype (zeroing the bucket counts, sum, and count while preserving the bucket bounds via 0 <$ histBuckets), with one change: it uses atomicModifyIORef' instead of modifyIORef' to match observe/observeAndSample and stay safe against concurrent observations.