equality of two array3s is true iff `a` and `b` have the same dimensions and for all
`i in indices0`, `j in indices1` and `k in indices 2` we have `a[i][j][k] = b[i][j][k]`.

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.