fromConstruct: Avoid recipK for a large speedup

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Clarify that deconstruct yields a Rational if possible

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Release version 0.1.0.1

Signed-off-by: Anders Kaseorg <andersk@mit.edu>