Clearly differentiate functions that accept prefix vs. key

[omus]

I thought about using s3_purge_versions initially, but that purges versions of everything under the prefix, rather than just a specific path---and I very intentionally just want to be touching the specific path here.

Oh my b, I assumed you could provide a full path as the prefix to s3_purge_versions.

We should make an issue to make it clear on a convention for what functions deal with keys and which deal with prefixes. Right now it isn't clear and accidental usage of a prefix function when you though a key was used could be quite dangerous.

Originally posted by @omus in #299 (comment)

We should review the AWS S3 API before making a decision on what we want to do but my initial instinct is that we should have a _prefix suffix on functions that accept prefixes and not for functions that use keys. We could also make use of dispatch with additional types.

[omus]

One option I like is using the _object suffix for actions on a single single S3 key/object and _prefix suffix for path prefixes.

For a moment I thought _objects could work for path prefixes but as that's a typo away from disaster I would recommend against it (e.g. s3_delete_objects(bucket, key))