Start Conway Imp tests with an initial committee and constitution

[teodanciu]

Description

This PR is setting an initial committee and constitution for Conway (emulating the ones that will be defined in genesis).
For this, it was necessary to slightly modify the initialization of the NewEpochState in the Imp framework, to allow initialization actions before returning the state.

Tests were adjusted to only elect a committee if this action is actually meaningful to the test (relying on the initial committee otherwise).
Going through many tests was an opportunity to make some small improvements:

• an attempt to be more disciplined with setting pparams. I have consolidated a few of them
• updating some functions to reduce the noise of converting between KeyHash and Credential
• removing the duplicated inclusion of a test

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]

Looks good, but committee initialization is a bit hacky and it seems that thresholds got relaxed for many tests

[teodanciu]

• Removed global bindings for committee, and generated them with freshKeyPair instead
• Set voting thresholds when initializing and adjusted the tests accordingly
• Added initial constitution

If you want to have another look @lehins when you can, thanks!

[lehins]

Looks great. Thank you!