feat!: implement default handling for fields

[maxveldink]

Defaults in Structs didn't work at all and failed the required field validations. This change makes these work by significantly changing the underlying Field class.

• We now unwrap nilable types in Fields, storing the non-nilable component in the type ivar and setting the field to optional.
• You can now indicate if you want something optional through the Field initializer instead of indicating if it's required. This allows you to pass a non-nilable type but benefit from optional validations.
• We now allow you to set a default to be used during coercion and validation if nil is supplied to an optional field.
• Refactored the StructCoercer to recursively call deserialize from a hash (sorry @antoinesaliba 😓 )