Better support for pluggable data models and better handling of side effects

[phochste]

Would like to see a way to e.g.

Support ways to plugin your own hash implementation (e.g Tie::IxHash, or any favorite implementation)

Support ways to add side-effects when executing fixes (e.g. logging, error handling, exports if individual records instead of the complete iterator).

(read monads)

[phochste]

Available in experimental 'monad' branch.

[phochste]

Available as Catmandu::Fix::Bind