equality of two arrays2 is true iff `a` and `b` have the same dimensions and for all
`i in indices0` and `j in indices1` we have `a[i][j] = b[i][j]`.

For performance, this will compare the length of the arrays first.

This is defined only if T : property.equatable, it will result in a compile-time panic
if this is not the case.