Extend `Foldable` class with `length` and `null` methods

This completes the `Foldable` class by two important operations which
this way can be optimised for the underlying structure more easily.

A minor fix for the `containers` submodule was needed to due name clash

Addresses #9621

Reviewed By: ekmett, dfeuer, austin

Differential Revision: https://phabricator.haskell.org/D250

1 files changed, 0 insertions, 0 deletions

diff --git a/libraries/containers b/libraries/containers
-Subproject e84c5d2145415cb0beacce0909a551ae5e28d39
+Subproject 085e1b8b2cfbd1159bbc9f8cbf6a4127cc32227