Add load function

[msanterre]

What this does

• Adds a load function so we can make a new hyperloglog directly from the bytes instead of needing the gob format.

Why is this needed?

I needed to deserialize HLLs that were serialized from another language.

#14 also tried to address this issue, but lacked tests and seemed to be abandoned.

Instead of setting a state, this returns a new HLL, which I think is cleaner.