Fix: Unable to shrink mapped command

[dubzzz]

No description provided.

[coveralls]

Coverage remained the same at 96.602% when pulling 21864f3 on fix/shrink-mapped-cmds into f9f6f41 on master.

[dubzzz]

Thanks to this commit it is now possible to shrink mapped commands like:

fc.nat().map(v => new MyCommand(v))

Before that commit, the shrinker would not have tried to shrink on the natural number generator - fc.nat.