Add a mode similar to Go's singleflight

[aromatt]

Go's singleflight (https://pkg.go.dev/golang.org/x/sync/singleflight). This library ensures that there is only one in-flight execution allowed at a time. This is similar to fnl's queued mode, except that

If a duplicate comes in, the duplicate caller waits for the original to complete and receives the same results.

Our queued mode just performs all calls in order; it does not try to reuse results.

This would make a nice addition to fnl's feature set.