Add assert(predicate) syntax

[TimWSpence]

Enables IO.pure(1).assert(_ > 0) syntax

[TimWSpence]

This was something that seemed useful to me earlier. Feel free to discard if you disagree 😅

[TimWSpence]

@armanbilge do I need to do anything with this? Or is it even wanted? 😂

[danicheg]

Thanks for this work, it's a lovely addition to the library.

[TimWSpence]

Thanks for this work, it's a lovely addition to the library.

Thanks! :)