Replace the single-iteration foreach loops

[scottdorman]

The single-iteration foreach loops in Requires.NotNullOrEmpty(IEnumerable values, string? parameterName) and Requires.NotNullOrEmpty<T>(IEnumerable<T> values, string? parameterName) can be replaced by a direct call to GetEnumerator().MoveNext(). This results in a slight performance boost and fewer IL instructions generated.