Ensure units are all created with matching tags

[ChrisPenner]

Overview

Values which should have been equal would be reported as non-equal WHEN:

• The matching values contained a unit value ()
• One unit value was created with a literal () and the other came from a builtin
• The units were in an Any when they were being compared
• They were compared using Universal.eq

The reason is that unit values were created with PackedTag 0 in some locations and unitTag in others;
the difference was only surfaced when wrapped in Any because universal equality within Any checks type tags.

Implementation notes

• Use the unitTag for all units regardless where they come from.
• Do the same for tuples (a.k.a. pairs)
• Clarify the use of PackedTag 0 for the pure effects case.

Test coverage

Ran all the base tests.

[ChrisPenner]

Just ormolu doing its thang

[dolio]

Looks good.

[ceedubs]

I can confirm that this fixes #5532 (which Chris probably already checked, but it isn't linked here).