Enumerable helpers

[Igorbek]

This task is to implement the following helpers for Enumerable:

internal static class EnumerableExtensions
{
    public static IEnumerable<T> WithElement<T>(this IEnumerable<T> source, T element) { ... }
    public static IEnumerable<T> WithoutElement<T>(this IEnumerable<T> source, T element, IEqualityComparer<T> comparer = null) { ... }
}

• WithElement must return sequence with added passed element
• WithoutElement must return sequence with added passed element
• both methods allow to accept null as a source, that means the collection is empty
• they must return non-null always
• implementation must not use List or Array or anything buffering. use yield return/yield break
• [new] method accept optional equality comparer of type IEqualityComparer<T>
  • if comparer is not specified or null passed, use default comparer from EqualityComparer.Default

[Igorbek]

Usage:

IEnumerable<int> seq = ...
IEnumerable<int> seq = seq.WithElement(10).WithoutElement(1);

[Igorbek]

@yuriyoleynikov task has been updated

[Igorbek]

implemented in #20