Fix melt bugs

[tranma]

this fixes two melt/constructor/deadcode bugs:

• don't replace prim_eq unit unit with unit (needs a default case for fold1)
• when we have a binding that doesn't get used and is not removed by dead code, melt will not melt it and so constructor won't get rid of it, and we are left with an unmelted value in the end (see Unmelted value in avalanche program #500)

ideally we should have a pass right after FromCore that freshify every name

! @amosr @jystic
/jury approved @amosr @jystic

[amosr]

👍

[jacobstanley]