Fix MarshalJSON not working for values

[lennart6443]

Marshalling a non pointer BitSets results in an empty JSON object:

var bs bitset.BitSet
bs.Set(0)
bytes, _ := json.Marshal(bs)
fmt.Println(string(bytes))

To fix this the BitSet.MarshalJSON method needs to be a value receiver instead of a pointer receiver.

[lemire]

This seems to match the documentation: https://pkg.go.dev/encoding/json#RawMessage.MarshalJSON