Fix bug in coshadow
Expand permutation statistics as sums of patterns
Introduce convenience functions stat, contains and avoids

Also rename previous function avoids as avoidsAll, thus breaking the
API. Bump version to 0.7
Update tests

The reason for this that the renam avoids -> avoidsAll broke some of
the tests
Implement (the inverse of) the Simion-Schmidt bijection
Fix typos
Refactor lMaxima and components
Refactor unrankLehmercode
Implement simionSchmidt and refactor its inverse
Test simionSchmidt
Implement the remaining principal classes Av(p) with |p| = 3
Lean direct sum and skew sum the other way
Define inc and dec classes
Improve documentation and naming
Make Pattern a typeclass.

This is to prepare for vincular, bivincular and mesh patterns, etc.
'newtype' instead of 'type' for Perm instances, except for String