Added EnsureExtensions and implementation of IEquatable<Range<T>>

https://www.nuget.org/packages/Platform.Ranges/0.0.3

Added EnsureExtensions for range-related exceptions.
Added implementation of IEquatable<Range> to Range struct.
Exception check from the constructor of Range struct moved to Ensure extensions so its logic can be reused now.
Added more comments to Range struct.
Two issues closed.