revert class_array and class_matrix

[t-kalinowski]

This PR removes class_array and class_matrix, pending further design and discussion.

#401 (comment)

array and matrix are unique because these are implicit S3 classes based on the presence of a dim attribute.

Quoting from ?UseMethod:

If the object does not have a class attribute, it has an implicit class. Matrices and arrays have class "matrix" or "array" followed by the class of the underlying vector. Most vectors have class the result of mode(x), except that integer vectors have class c("integer", "numeric") and real vectors have class c("double", "numeric").

E.g.:

> x <- 1
> attr(x, "dim") <- c(1, 1)
> .class2(x)
[1] "matrix"  "array"   "double"  "numeric"

[lawremi]

I feel like this might have jumped the gun a little. An S7_S3_class for matrix and array is still useful, because there are objects with that class. Sure, the hierarchy varies, but we can still use these for dispatch and properties.

To make something responsive to is.matrix() is orthogonal to these classes, in my opinion.

[hadley]

@lawremi I think we just need to discuss this a bit more before committing to an API. And we want to do a minor release soon, so pulling these out just gives us a bit more time to think about them.