Allow initial elements as variadic into NewOrderedMap

[pd93]

Closes #43

Adds a variadic argument to NewOrderedMap which allows the user to specify a set of initial data when initialising an ordered map. In my initial proposal I suggested a new function called NewOrderedMapWithElements. However, this isn't necessary to maintain backwards compatibility since the argument is variadic and still works when no arguments are set.

I also considered a functional options style API which would allow for greater compatibility with any future changes:

type OrderedMapOption[K comparable, V any] func(*OrderedMap[K, V])

type OrderedMap[K comparable, V any] struct {
	kv map[K]*Element[K, V]
	ll list[K, V]
}

func NewOrderedMap[K comparable, V any](opts ...OrderedMapOption[K, V]) *OrderedMap[K, V] {
	om := &OrderedMap[K, V]{
		kv: make(map[K]*Element[K, V]),
	}
	for _, opt := range opts {
		opt(om)
	}
	return om
}

func WithInitialData[K comparable, V any](els ...*Element[K, V]) OrderedMapOption[K, V] {
	return func(m *OrderedMap[K, V]) {
		for _, el := range els {
			m.Set(el.Key, el.Value)
		}
	}
}

However, this may be overkill for what is currently quite a simple API. It also loses the ability to initialise the underlying map with the correct size.

We could consider setting the size of the underlying list too, but I assume you are not doing this currently to save allocations. Maybe we only initialise if len(els) > 0?

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This change is 

[elliotchance]

Thanks @pd93 !