(#16509) Levity-polymorphic join points in GHCi
Commit message:
GHCi support for levity-polymorphic join points
Fixes #16509.
See Note [Levity-polymorphic join points] in ByteCodeGen,
which tells the full story.
This commit also adds some comments and cleans some code
in the byte-code generator, as I was exploring around trying
to understand it.
test case: ghci/scripts/T16509