fix #527

Use []uint32 in bitset instead of []uint64, because it's harder to guarantee
64-bit alignment of []uint64 than I had realized:

https://go101.org/article/memory-layout.html

irc/utils/bitset_test.go

@@ -5,7 +5,7 @@ package utils
 
 import "testing"
 
-type testBitset [2]uint64
+type testBitset [4]uint32
 
 func TestSets(t *testing.T) {
 	var t1 testBitset