Add Validation

[commonsensesoftware]

Adds service validation which covers the following main scenarios:

• A missing, required, dependent service
• A circular dependency (which results in stack overflow)

ServiceCollection::build_provider() should not allow an invalid ServiceProvider to be created. The signature will, therefore, now be:

pub fn build_provider(&self) -> Result<ServiceProvider, ValidationError>() { }

Since that is a breaking change, the crate will also bump to 2.0.