accounts/abi: fix event unpack into slice

+ The event slice unpacker doesn't correctly extract element from the
slice. The indexed arguments are not ignored as they should be
(the data offset should not include the indexed arguments).

+ The `Elem()` call in the slice unpack doesn't work.
The Slice related tests fails because of that.

+ the check in the loop are suboptimal and have been extracted
out of the loop.

+ extracted common code from event and method tupleUnpack

accounts/abi/argument.go

@@ -49,3 +49,13 @@ func (a *Argument) UnmarshalJSON(data []byte) error {
 
 	return nil
 }
+
+func countNonIndexedArguments(args []Argument) int {
+	out := 0
+	for i := range args {
+		if !args[i].Indexed {
+			out++
+		}
+	}
+	return out
+}