Reimplement constraint as class

[waalge]

Describe the feature you'd like

I like the idea of using constraints. I really want to use statemachines. But I don't understand numerous bits of the code.

FR : Implement Constraint as a class and allow me to write my own instances.

Describe alternatives you've considered

For example https://github.com/input-output-hk/plutus-apps/blob/37c54aac5e2c6f765a6fff2503447b8ad0c0d259/plutus-ledger-constraints/src/Ledger/Constraints/OnChain.hs#L117
Its pretty suggestive that the mustPayToScriptAddress constraint does actually check the staking address. It doesn't seem to.
4 lines later choice of variable name svh is suggestive that its stake validator hash but its actually the datum hash.
There's convoluted logic around minAda which is likely well motivated but given that minAda is hard coded to 2 Ada ... this is gonna be pretty hit or miss on-chain with multiple native assets.

Can i fix this without maintaining my own fork? At the moment I don't think so.

Would fixing this be the end of this issue? Likely not.
As with the introduction of TxConstraintFun (I assume), there will be logic expressible as constraints, generally, but not expressible by an existing fixed list.