A Vinyl statement may be either a key or a tuple. We must use different
functions for the two kinds when working with a bloom filter. Let's
introduce helpers incorporating that logic.

Notes:
 - Currently, we never add keys to bloom filters, but after the next
   patch we will, so this patch adds tuple_bloom_builder_add_key helper.
 - According to the function protocol, tuple_bloom_builder_add may fail
   with out-of-memory, but we never checked that. Fix that while we are
   at it.