v4.0.2 ROLLBACK - Reverting change to Seq1

@StefanBertels has rightly highlighted some possible edge-cases with yesterday's rollout. These edge cases could turn into very real bugs in your application, and so I am:

• Rolling back the change to Seq1: Seq1 will continue to be the way you create singleton Seq<A> collections.
• Keeping the migration of the Seq conversion functions to toSeq. This will be consistently be a compilation error, which you can fix up by just renaming the Seq(blah) to toSeq(blah) or blah.ToSeq().

It isn't ideal that Seq1 is still hanging around, being all inconsistent with the rest of the library, but neither is putting a burden of tracking all possible edge cases. And so, this is the right thing to do.

What I will do is pause the migration of Seq1 to Seq until June 2022. That should give everyone enough time to get to version 4 and migrate their Seq conversion function usage to toSeq. Then the subsequent change will have zero impact.

For those that don't upgrade in that time and want the safest possible migration, they can upgrade to a pre-June 2022 version, fix the conversion functions and then upgrade to the latest.

Apologies to anyone who started the migration yesterday.