Replace ssz with ssz2, adapt ssz_derive

eth2/utils/ssz_derive/tests/tests.rs

@@ -0,0 +1,22 @@
+use ssz_derive::Encode;
+use ssz::Encodable;
+
+#[derive(Debug, PartialEq, Encode)]
+pub struct Foo {
+    a: u16,
+    b: Vec<u8>,
+    c: u16,
+}
+
+#[test]
+fn encode() {
+    let foo = Foo {
+        a: 42,
+        b: vec![0, 1, 2, 3],
+        c: 11,
+    };
+
+    let bytes = vec![42, 0, 8, 0, 0, 0, 11, 0, 0, 1, 2, 3];
+
+    assert_eq!(foo.as_ssz_bytes(), bytes);
+}