Define YANG cardinality extensions

We are reusing leaf nodes for well-known-low-cardinality types (YANGTOOLS-494). In order to cover more types we need a YANG extension which can be used to hint cardinality.

There are three extensions we need:

• cardinality:exact, where the set of possible matching values for a type is known at design time. This can be attached to a leaf, leaf-list and typedef. The size of the set should be specified as the argument.
• cardinality:low, where the set size is known, but expected to be low – such that it can be used with LRU interners. This can be additionally attached to container, input, output, notification, case, choice and list statements.