Add missing result combinators

[jmid]

Fixes #303

This PR adds result combinators to

• QCheck as an arbitrary
• QCheck.{Gen,Print,Shrink,Observable}, and
• QCheck2.{Gen,Print,Observable}.

Shrinking-wise they just reduce within Ok _ and Error _ without attempting to reduce one to the other.

Edit:
Ratio-wise, I've gone with 75% in favor of generating Ok _ - compared to option's 85% preference for Some _,
from the rationale that Ok _ is the preferred and expected path/value, but with a greater chance of Error _ since its constructor is more expressive than None and hence should be exercised more.