Look for more places to add Collectors

[cpovirk]

It might be interesting to search our codebase for which Guava methods are called most often on the result of collect(toList()) and so forth. For example, I got to wondering whether Futures.whenAllSucceed/whenAllComplete is often used that way, in which case we might want to offer collect(whenAllSucceed()).call(...) (and maybe we'd then consider renaming call to thenCall?). (It looks like maybe on the order of 5% of all calls, which is not bad when you consider that we haven't had streams that long -- results saved internally as whenall-stream. [Edit: I estimate that roughly 2/3(!) of whenAll* calls were written after streams became available.])