Move balancer to a separate package with public API

[euonymos]

Some dApps may need to use specific balancing strategies that cannot be expressed using balancing constraints. In Cardano balancers can utilize different strategies most of which are designed to prevent dust, and they have their pros and cons. One example is a wallet used by a backend service to perform some administrative operations that we want to run in parallel. The ability to provide a custom strategy looks very attractive for cases like that.

[klntsky]

True. There are at least two ways forward I see:

• do not use CTL's balancer, implement your own. It's actually not that hard, you can repurpose CTL's guts
• propose an interface for pluggable balancers

I'd vote in favor of decoupling balancer logic from CTL altogether, just like we did with tx builder: https://github.com/mlabs-haskell/purescript-cardano-transaction-builder
however, balancer needs to query both the backend and the wallet, so it's not that easy...