Explain the reason for this library to exist

Why would you use this library when there is a perfectly good existing solution in https://github.com/Keats/validator ?

+ Asynchronous validations (that may require a call out to an external API)
+ By implementing the formatting as a function, you can perform type-safe formatting and localization of the values using something like https://crates.io/crates/i18n-embed-fl
+ I try to avoid macros where possible, this API is designed to be easy to use without macros
+ Composable and flexible validators

Review the issue text and its comparison with Keats/validator first. Document why this library exists, covering asynchronous validation, type-safe formatting and localization, avoiding macros, and composable validators; done when those motivations are clearly explained.

Written by the indexing model from the issue text.