Improvements on Tweaks

[mmontin]

This PR proposes a collection of small improvements:

• added a Tweak to remove a label
• factored the Tweak that adds a label
• added a Tweak to check if a label is present
• created a file to store Tweaks working on labels
• factored txSkelOutValidators
• homogenized negativePart and positivePart by relying on Pl.negate
• renamed overMaybeTweakSelecting to overMaybeSelectingTweak for consistency
• added an optic to filter foci
• added a new permutation tweak which picks a random permutation and applies it to outputs
• added tests to that new Tweak.
• harmonized names for both possible permutation (all or single)

[mmontin]

I changed the implementation of the random permutation in two ways:

• it is now possible to provide a seed to change the outcome of the permutation process on the same input
• the tweak that makes this permutation will fail if the result is unchanged This satisfies, I believe the requirement for this Tweak; that is to give a pseudo-random way of permutating the outputs of a transaction to test to resilience of contract for such variations. I believe this PR is mergeable following these changes.