add pgx.CopyFromSlice

Using CopyFromRows can often be inconvenient to use, because you would
need to convert a typed array to an [][]interface{}. Similarly,
implementing a custom CopyFromSource is too verbose for one-off things.

Add CopyFromSlice that allows to more easily convert a slice to a
CopyFromSource. Example:

    copyCount, err := conn.CopyFrom(
        context.Background(),
        pgx.Identifier{"people"},
        []string{"first_name", "last_name", "age"},
        pgx.CopyFromSlice(len(rows), func(i int) ([]interface{}, error) {
            return []interface{user.FirstName, user.LastName, user.Age}, nil
        }),
    )

copy_from_test.go

@@ -69,6 +69,65 @@ func TestConnCopyFromSmall(t *testing.T) {
 	ensureConnValid(t, conn)
 }
 
+func TestConnCopyFromSliceSmall(t *testing.T) {
+	t.Parallel()
+
+	conn := mustConnectString(t, os.Getenv("PGX_TEST_DATABASE"))
+	defer closeConn(t, conn)
+
+	mustExec(t, conn, `create temporary table foo(
+		a int2,
+		b int4,
+		c int8,
+		d varchar,
+		e text,
+		f date,
+		g timestamptz
+	)`)
+
+	tzedTime := time.Date(2010, 2, 3, 4, 5, 6, 0, time.Local)
+
+	inputRows := [][]interface{}{
+		{int16(0), int32(1), int64(2), "abc", "efg", time.Date(2000, 1, 1, 0, 0, 0, 0, time.UTC), tzedTime},
+		{nil, nil, nil, nil, nil, nil, nil},
+	}
+
+	copyCount, err := conn.CopyFrom(context.Background(), pgx.Identifier{"foo"}, []string{"a", "b", "c", "d", "e", "f", "g"},
+		pgx.CopyFromSlice(len(inputRows), func(i int) ([]interface{}, error) {
+			return inputRows[i], nil
+		}))
+	if err != nil {
+		t.Errorf("Unexpected error for CopyFrom: %v", err)
+	}
+	if int(copyCount) != len(inputRows) {
+		t.Errorf("Expected CopyFrom to return %d copied rows, but got %d", len(inputRows), copyCount)
+	}
+
+	rows, err := conn.Query(context.Background(), "select * from foo")
+	if err != nil {
+		t.Errorf("Unexpected error for Query: %v", err)
+	}
+
+	var outputRows [][]interface{}
+	for rows.Next() {
+		row, err := rows.Values()
+		if err != nil {
+			t.Errorf("Unexpected error for rows.Values(): %v", err)
+		}
+		outputRows = append(outputRows, row)
+	}
+
+	if rows.Err() != nil {
+		t.Errorf("Unexpected error for rows.Err(): %v", rows.Err())
+	}
+
+	if !reflect.DeepEqual(inputRows, outputRows) {
+		t.Errorf("Input rows and output rows do not equal: %v -> %v", inputRows, outputRows)
+	}
+
+	ensureConnValid(t, conn)
+}
+
 func TestConnCopyFromLarge(t *testing.T) {
 	t.Parallel()