Maps where every possible key has an associated value.
Only entries with uncommon values are actually stored in the map; all other keys are presumed to
be associated with a common value. The definition of "common" and "uncommon" can be customized via
the Commonality
trait.
num-traits
: provides a commonality implemented in terms ofnum_traits::Zero
.serde
: providesserde::Serialize
andserde::Deserialize
implementations.