AbstractIterable: Keep a reference to the underlying iterables. See ceylon/ceylon-compiler#1351
AbstractIterable: Figure out an initial size for the SequenceBuilder when
converting to a Sequence, at least in those cases where we can compute that size
without having to iterate over the elements. If the comprehension contains
if clauses, apply an upper limit to avoid having huge arrays backing very small
sequences due to highly selective filters. See ceylon/ceylon-compiler#1337