add `lazyMerge(..)` to complement `lazyZip(..)`

[getify]

lazyZip(..) currently works like this:

for await (let [q1NextVal,q2NextVal,q3NextVal] of RevocableQueue.lazyZip(q1,q2,q3)) {
   // ..
}

To complement this helper, add lazyMerge(..):

for await (let nextVal of RevocableQueue.lazyMerge(q1,q2,q3)) {
   // ..
}