Make Seq.to_array only traverse seq once
This PR suggests a change to `Seq.to_array`, which uses construction of
an intermediate list to prevent traversing the Seq twice. This requires
the allocation of an intermediate list, but it eliminates the surprising
behavior that otherwise occurs with state-full sequences, due to the
extra traversal required to obtain the length of the sequence. E.g.,
with the previous implementation, the value of `to_array` for the
sequence constructed in the test added in this commit is `[|4;5;6|]`,
while `to_list` gives `[|1;2;3|]`.
