Add validation based on restriction

[LeonidKrutovsky]

<xs:simpleType name="Name">
  <xs:restriction base="xs:string">
    <xs:maxLength value="64"/>
  </xs:restriction>
</xs:simpleType>

[victor-soloviev]

For this structure we could generate following code (using https://github.com/Keats/validator):

#[derive(Default, PartialEq, Debug)]
pub struct Name (String);

impl Validate for Name {
    fn validate(&self) -> Result<(), ValidationErrors> {
        if self.0.len() > 64 {
            let mut errors = ValidationErrors::new();
            errors.add("Wrong name length", ValidationError::new("Name is too long"));
            return Err(errors);
        }
        Ok(())
    }
}

Code above could be tested with this:

let mut name: Name = Default::default();
    name.0 = "...".to_string();
    match name.validate() {
        Ok(_) => println!("ok"),
        Err(e) => println!("{}", e)
    };

Note that

• we could not use existing #[validate(length(max = 64))] for type Name. This option is not supported by the Keats/validator;
• we could not use #[validate(custom = "...")] for type Name (actually, we could, but it requires Serde to be derived; we don't want this, since we use Yaserde);
• to implement the trait is the only option in this case.

We could create a macros that generates impl Validate for us. We want something like that:

#[derive(Default, PartialEq, Debug, Validate)]
#[ValidateLengthMax = 64]
pub struct Name (String);