Update Hasher: add input checking, amend helpfile

[mossheim]

In response to #2588.

I changed the summary because technically a hash function involves no randomness. I did leave it in the category UGens>Random because that's a pretty good use case; i.e., most people wouldn't actually use this UGen as an actual hashing algorithm.

[nhthn]

As we discussed, another bug is that Hasher.ar doesn't check the rate of its inputs (Hasher.ar(0) outputs garbage and should be caught at the client). Please add this method:

checkInputs {
    if(rate == \audio) { ^this.checkSameRateAsFirstInput; };
    ^this.checkValidInputs;
}

[mossheim]

Thanks! Didn't know the fix was that simple.