use functional interface for buff-extensions

[reset]

Work in progress. Tied to:

• Refactor extensions with a more functional approach buff-extensions#4
• Replace global Monkeypatching with Refinements buff-extensions#5

[sethvargo]

@reset is this still required?

[reset]

@sethvargo it's not required but the underlying library should still be improved. We should avoid monkey patching always and just use functional interfaces like this.

[sethvargo]

Agreed 😄