Add Enum Ratio to PlutusTx stdlib. Remove Ratio: half/toGHC/fromGHC

[bezirg]

Added an Enum Rational instance that mimicks that of GHC and Haskell report.

I also removed the toGHC/fromGHC from plutustx since they don't work on-chain.
I think it is straightforward to re-create these two-functions for off-chain use, simply by a combination of numerator and denominator.

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[SeungheonOh]

Changes look good, but I think it's okay to keep fromGHC and toGHC as is. We have other helpers in plutus-tx and plutus-ledger-api that won't work on-chain but is quite useful.

[bezirg]

Changes look good, but I think it's okay to keep fromGHC and toGHC as is. We have other helpers in plutus-tx and plutus-ledger-api that won't work on-chain but is quite useful.

I thought we are trying to keep plutus-tx on-chain only. But I am open to discussions about this and if so , bring it back.
If it is the case to keep it then I propose to change the name fromGHC to something more generic like fromHS or fromHaskell.

[SeungheonOh]

I thought we are trying to keep plutus-tx on-chain only.

I was mostly about keeping plutus-core out from packages that defines plinth types. I don't have a firm opinion on things that interfaces with regular Haskell as they don't often add any new dependencies anyways