Type Alias EntityResolver 

pub type EntityResolver<'input> = dyn Fn(Option<&str>, &str) -> Result<Option<&'input str>, String> + 'input;

Function to resolve external entities

This function is passed the optional public ID and the mandatory URI of the external entity.

It is expected to yield the content defining the URI, possibly preceeded by a text declaration.

If it yields None, the entity is discarded and will yield errors if referenced during parsing.

Errors must be stringified so they can be propagated via Error::EntityResolver.