Returns the size/length of a finite structure as an Int. The
default implementation just counts elements starting with the
leftmost. Instances for structures that can compute the element count
faster than via element-by-element counting, should provide a
specialised implementation.
The genericLength function is an overloaded
version of length. In particular, instead of returning an
Int, it returns any type which is an instance of Num. It
is, however, less efficient than length.