Tracking issue for closures in futures 0.4 combinators

[taiki-e]

This is a tracking issue for consistency in APIs that use closures in futures 0.4.

For individual cases, see below. (Especially #1889, #2470, and #2518 have some discussion on this)

• Should closures of StreamExt::map and TryStream::map_{ok, err} return a Future? #1889
• Why does StreamExt::filter and StreamExt::filter_map require functions that returns futures #1987
• Missing synchronous version of StreamExt::filter_map #2237
• Should TryStream::try_filter accept closures that return Results? #2375
• Review TryFutureExt combinators #2470
• Various inconsistencies around sync/async in TryStreamExt and StreamExt #2518

(I created this issue to avoid duplication of discussion between them.)

In most cases, the APIs where people are unhappy seem to be filter-related. (Note that this does not necessarily mean that "many people are unhappy with the current filter-related API" since those who are not unhappy with the current API will not leave comments.)