Zero-cost-wraps usize-s to give them a specific type. The motivation is to have different kinds of
indices that are incompatible at type-level, thus lowering the chance of mixing them up compared to
using usize-s.
Index-type creation is done through a macro, so the type actually belong to the client crate. This lets users augment index-types with methods, trait implementations, etc.
See the documentation for details.
If you are experiencing problems upgrading from a version < 0.9.17, make sure you read the
changelog.