Add additional reified list utilities

[poteat]

• Reify String.ToList to a value-level function.
• Add List.FindIndex to find the index of a value in a list that satisfies a predicate.
• Add List.IndexOf to find the index of a value in a list.
• Add List.StartsWith to check if a list starts with a sequence of values.
• Add List.EndsWith to check if a list ends with a sequence of values.
• Add List.IndexOfSequence to find the index of a sequence of values in a list.
• Fix runtime list search utilities to search via deep equality.
• Add List.Replace to replace all instances of a value in a list with another value.
• Add List.Remove to remove all instances of a value from a list.
• Add List.ReplaceSequence to replace all instances of a sequence of values in a list with another sequence.
• Add List.RemoveSequence to remove all instances of a sequence of values from a list.
• Fix NaturalNumber.decrement to return zero when decrementing zero during runtime.
• Add Combinator.Fix to find a fixed point of a higher-order type.
• Reify List.MinBy to a value-level function.
• Add List.MaxIndexBy to find the index of the maximum element in a list according to a scoring function.
• Add List.MinIndexBy to find the index of the minimum element in a list according to a scoring function.
• Add List.RemoveIndex to remove an element at a specified index from a list.
• Reify List.Reduce to a value-level function.