GetImplicit3: Replace (/=) with pattern match

Previously (/=) demanded an instance for Eq (a -> b). Presumably this
used to be provided by NumInstances until 1.3
ExtOpenScad/Definitions: Remove Show ArgParser instance

This required Show for functions
HACK: Add Show (a -> b) instance

NumInstances dropped this instance. In order to get things compiling
again we add it locally. This is terrible however and really needs to be
fixed properly, probably by relying less on Show.
Merge pull request #101 from bgamari/master

A few NumInstances fixes