refactor-for-testability (#3)

Co-authored-by: jdl <jdl@desktop>
Co-authored-by: jdl <jdl@crumpington.com>
Reviewed-on: #3

peer/bitset.go

@@ -0,0 +1,21 @@
+package peer
+
+const bitSetSize = 512 // Multiple of 64.
+
+type bitSet [bitSetSize / 64]uint64
+
+func (bs *bitSet) Set(i int) {
+	bs[i/64] |= 1 << (i % 64)
+}
+
+func (bs *bitSet) Clear(i int) {
+	bs[i/64] &= ^(1 << (i % 64))
+}
+
+func (bs *bitSet) ClearAll() {
+	clear(bs[:])
+}
+
+func (bs *bitSet) Get(i int) bool {
+	return bs[i/64]&(1<<(i%64)) != 0
+}