MozPromise::All sets up 'Then' lambdas on all sub-promises, each one
taking the resolve/reject object by value. Since this value will not be
used again in the lambda, it is safe to Move it, and from there
MozPromiseHolder::Resolve/Reject can also Move it again into the holder
storage, potentially saving two copies per Resolve/Reject. Also, once
all sub-promises have been resolved, the resolve-values can be Move'd
into the joining promise's Resolve function.
