IndexMap is a hash table where the iteration order of the key-value
pairs is independent of the hash values of the keys.
IndexSet is a corresponding hash set using the same implementation and
with similar properties.
This version of indexmap requires Rust 1.18 or later.
The indexmap 1.x release series will use a carefully considered version upgrade policy, where in a later 1.x version, we will raise the minimum required Rust version.
A hash set implemented using
Key equivalence trait.