Add devel/go-cmp Version 0.4.0

go-cmp is a package for equality of Go values.

This package is intended to be a more powerful and safer alternative
to reflect.DeepEqual for comparing whether two values are semantically
equal.

The primary features of cmp are:

- When the default behavior of equality does not suit the needs of the
  test, custom equality functions can override the equality
  operation. For example, an equality function may report floats as
  equal so long as they are within some tolerance of each other.

- Types that have an Equal method may use that method to determine
  equality. This allows package authors to determine the equality
  operation for the types that they define.

- If no custom equality functions are used and no Equal method is
  defined, equality is determined by recursively comparing the
  primitive kinds on both values, much like reflect.DeepEqual. Unlike
  reflect.DeepEqual, unexported fields are not compared by default;
  they result in panics unless suppressed by using an Ignore option
  (see cmpopts.IgnoreUnexported) or explicitly compared using the
  AllowUnexported option.

(nikita)