Allow opt-out of unsafe methods

[japgolly]

Opt-out of

• setState
• modState{,O,U}
• state ← technically but this can stay
• ==>
• -->

Add:

• IO[S]
• S ⇒ IO[Unit]
• S ⇒ S ⇒ IO[Unit]
• Versions of above with ChangeFilter

[japgolly]

Reminder to future-me:

1. You're soooooooooooo good-looking. -Z
2. A good solution might be via modules: Define an Unsafe type. The unsafe module defines an implicit for Unsafe such that it's always satisfied. Could always go the other way and define 2 implicits in a safe module to prevent (by default) unsafety. ("unsafeness"?)

[japgolly]

Oh actually it would be best to be able to (at runtime) change schemes.
Tests I'd want unsafe.

[japgolly]

#145 supersedes this. We will soon have One Way (and it's not scalaz.IO).