eth/erc20_params: use packed struct

This reduces the space efficiency.

Before: 756400 bytes
After: 749440 bytes
Diff: -6960 bytes

Author: benma

src/rust/bitbox02-rust/src/hww/api/ethereum/erc20_params.rs

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#[repr(packed)]
 struct P {
     pub unit: *const u8,
     pub contract_address: [u8; 20],