Fix too many discards.

[TimSheard]

Figure out how to get non-empty poolDistr from "random" SnapShots.

The test "Bruteforce = Pulsed, in every epoch, on traces of length 150"
Some times fails because at the epoch boundary because "There are no stakepools to choose an issuer from"
We set this up to discard, but occasionally we reach 150 discards and the test fails.
A lot of analysis discovers that in the random initial NewEpochState, the mark snapshot is not well formed, in that when ones computes the PoolDistr from such a snap shot, PoolDistr is empty.

The fix is to arrange random snapshots such that they never result in an empty PoolDistr. So we had to change the constraints for 'regPools' , 'incrementalStake', and 'delegations' so this never happens. In order to do that
we had to change the order we solve constraints, so that 'utxo' and 'incementalStake' are solved before 'delegations' . To do that we created a new Stage utxoStage, in the module Preds/UTxO.hs,
so we can see the UTxO when specifying the DState and PState.

Description

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated. If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages. New section is never added with the code changes. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[lehins]

Thank you for fixing it!