[Fix #12] Introduce feature flags to make extension of io/Coercions optional

[vemv]

These allow dependent libraries to use fs without the side-effect of extend-protocol.

Otherwise one can unawarely create issues, as seen in https://github.com/clojure-emacs/clj-refactor.el/issues/508.

https://clojure.org/reference/protocols#_guidelines_for_extension says:

If you don’t own the protocol or the target type, you should only extend in app (not public lib) code, and expect to maybe be broken by either owner.

So what fs was doing wasn't ideal anyway.

The proposed feature flag is the best way I found such that:

• the existing behavior is preserved
• refactor-nrepl can effectively require this ns and modify the feature flag without affecting anyone else
  • this is made possible by https://github.com/benedekfazekas/mranderson - it 'inlines' (renames) the whole ns, so we end up only changing our copy of the fs namespaces.
• Regular apps can also change the value if desired
  • it's a little more risky for them because they don't have mranderson. But OTOH an app is more of "closed system" which can be quite exhaustively unit-tested.

Cheers - V

[slipset]

I think this is a valid approach, and seems to solve the downstream problem.

[vemv]

Cheers, worked nicely clojure-emacs/refactor-nrepl#356