unroll update-in, add update-in benchmarks, add -seq on vector benchmark.
if -seq is called on PersistentVector w/ cnt < 32, return IndexedSeq
for now. In theory ChunkedSeq should be as fast but it requires more
work. This a big perf boost anywhere -seq is called on
PersistentVector such as idiomatic use of apply with vectors and
update-in where the key list is written as a vector.