Implement Single#collectInOrder()

PR #186 added `collect()` methods for `Single`. These methods internally use `Publisher#flatMapSingle()` and hence does not guarantee order in the generated `Collection` (returned by `collect()`). As we do not have a method that preserves order (`concatMapEager()`) we can not provide a `collect()` variant that preserves order. It will be a useful API to provide.

Start by reviewing PR #186 and the existing collect() methods for Single. Compare their use of Publisher#flatMapSingle() with the ordering behavior described for concatMapEager(); done means providing a Single collection variant that preserves source order.

Written by the indexing model from the issue text.