Make 'unst' a method of Perm, and remove 'neutralize'
Simplify by generalizing from StPerm to arbitrary perms.

Previously, functions such as 'empty', 'one', '/-/' 'fromVector' and
'toVector' were specific to StPerm. They now work for any perms. This
also simplified the definition of a few other functions.
Define the class of separable permutations
A few simplifications, mainly to the Quickcheck-tests
Define inflations