Allow deriving Dummy with generic types

[andrewtoth]

Currently this fails to compile with a cryptic error message:

#[derive(Dummy)]
struct MyStruct<T> {
    field: Vec<T>,
}

missing generics for struct `MyStruct`
expected 1 generic argument

This adds support for deriving Dummy trait on types with generics, provided the concrete type will implement Dummy as well. Otherwise the error message is more friendly. eg for a type NoDummyImpl:

let g: MyStruct<NoDummyImpl> = Faker.fake();

the trait bound `NoDummyImpl: fake::Dummy<fake::Faker>` is not satisfied