Bug: PValue needs some outer unionWith, breaks PEq

[lemmon-714]

Right now it relies on AssocMap's unionWith, which drops keys that are not present in both maps. This breaks a couple of things:

• adding/subtracting/multiplying values
• PEq is wrong

[lemmon-714]

Might also be that I'm slightly incorrect here, double-checking rn. But in any case there's undefined behaviour without some way to specify default values.

[blamario]

You are slightly incorrect, see issue #556 and PR #558.

[lemmon-714]

So on my Plutarch version it's broken, verified it - sparing you the details until I unfuck my attempt to use nix. My version was from around August. Will let you know if it persists, but would assume you fixed it as you seemed aware of the issue.

[lemmon-714]

• I think I managed to update my Plutarch version haphazardly, probably. Very likely.
• I need to note that even in my old version the Eq wasn't broken, I raised this issue purely looking at the comments in the code, and wondering how punionWith # (plam (-)) was supposed to work without defaults. Which it does not, but it works well enough to implement Eq.