lib/array: fix AbstractArray::unshift

Signed-off-by: Jean Privat <jean@pryen.org>
lib: document abstract_collection + nitunit tests

Signed-off-by: Jean Privat <jean@pryen.org>
lib/coll: move *index_of* in Sequence

Signed-off-by: Jean Privat <jean@pryen.org>
lib/coll: generalize `insert` in Sequence

Signed-off-by: Jean Privat <jean@pryen.org>
lib/coll: make CoupleMapIterator private

Thus introduce a protected service `CoupleMap::couple_iterator` to
factorise the instantiation.

Signed-off-by: Jean Privat <jean@pryen.org>
lib/coll: make private some specific implementations of interfaces

Signed-off-by: Jean Privat <jean@pryen.org>
tests: do not use now-private classes

Signed-off-by: Jean Privat <jean@pryen.org>