Modernize pack layout

Switch to markdown syntax for README and create symlinks for GitHub
and PlDoc.
Migrate to TAP-style tests
Add multifile declaration for has_type/2

Without the declaration, loading the error module abolishes our
declaration and generates a warning.
Implement minimum and maximum predicates

It's often convenient to find the smallest or largest term in a list.
Without dedicated predicates, the easiest way is to sort the list and
choose the first (or last) element.  This requires more computation
than a simple list traversal.

These predicates are also convenient when working with library(func)
since they can be used as functions.
Bump version to 0.1.1