The GopherJS transpiler will converts

slice := []byte{1, 2, 3, 4}
array := [4]byte(slice)

into

array = $clone(($clone(slice, arrayType)), arrayType);

the $clone calls the copy method on the array type. The copy method used src.length that is undefined for a slice. I added to the array type copy method to also allow for conversion from slices into arrays. The secondary $clone is from the assignment of an array to an array.

This will fix the issues in net/netip where they are converting slices of different lengths into arrays for ip addresses. This change should fix the last issue in GopherJS Tests so it should pass. Most of CI should pass, but measurement_size.yml will still fail in this PR (it's fixed by another PR).

This is part of #1270