.find -> .filter?

[Doug Evans]

I see r6rs has find, filter, partition
where find returns the first match,
filter returns all matches,
and partition returns those that match and those that don't match.
[Partition uses the multiple-return-values feature.
I'm not suggesting we use that here.]

I think the thing to do is follow suit and rename .find to .filter.
I like consistent naming.

[Internally I'd also rename utils.scm's find-first and find,
but that's an internal detail.]