Serializable errors

Implement the Serialize trait on ValidationError and ValidationErrors, converting the flat vector of errors in the latter into a HashMap.

I needed serializability so that validation errors can easily be passed to the Tera template context to be displayed on web pages, grouped by the error keys.

Working fork here:
https://github.com/pzingg/form-validation/tree/serialize

Example application using the fork to insert ValidationErrors in Tear context here:
https://github.com/pzingg/rust-starterapp/tree/rust-2021

Start by locating the definitions of ValidationError and ValidationErrors, then review the linked serialize branch for the intended behavior. Implement serialization for both types, converting ValidationErrors' flat error vector into a HashMap grouped by error keys, and verify the result can be inserted into a Tera template context.

Written by the indexing model from the issue text.