monomial: initial code for the new degree methods.
symbol_set: add a small helper class to identify the positions of the symbols
of one set into another. To be used for the future partial degree methods.
array_key: add a small todo item.
symbol_set: add some minor functionality to the positions class.
monomial: some more progress towards the new degree methods.
small_vector: minor cleanup.
Shelve the univariate_monomial class for now, it's not really used anywhere
and it's behind feature-wise.
small_vector: some cleaning up for the templated enablers.
small_vector: drop the standard layout requirement.
small_vector: gather improvement ideas in list.
array_key: sanitize the type requirements, make them method-specific and beautify
the generic enablers. Also make the add() method public and change its name for
clarity.

Also minor change in monomial as response to the changes in array_key.
ak: fix typo in ze docs.
ak: small comment addition.
polynomial: fix a test after the removal of univariate_monomial.
monomial: a first batch of changes involving new-style template enablers.
monomial: some cleaning up for the linear_argument and pow methods.

Use the new-style enablers and change the pow enabler to work with safe_cast.
monomial: documentation and bits of testing for the new degree methods.
monomial: a couple of test additions.
monomial: another comment/todo cleanup.
monomial: minor doc cleanup.