Enforce consistency between the available operations of IterableOnce and Iterable

[julienrf]

Currently these collection types don’t have a common super type.

@lrytz suggested that we write some tests that use reflection to check that the operations and type signatures are consistent between them.

[lrytz]

Here's a the current diff of Iterable vs Iterator operations: https://gist.github.com/lrytz/39bdb1b25c23e204c3263809534ba6a0/revisions#diff-5b3b7587cc6e69d92ccb282bab852b31

[lrytz]

See also https://github.com/scala/collection-strawman/issues/419

[szeiger]

This sounds a lot like the IterableMonoTransforms we had in earlier iterations of the design.

[szeiger]

I had a look at the diff and most of what I saw looks reasonable. Iterator does not have methods like dropRight that would require forcing and caching everything. OTOH, because an Iterator represents a single traversal, it has some Seq-like features that are missing from Iterable.