Merge futures-util and futures-executor to main futures crate

[taiki-e]

I would propose to merge these utility crates to the main futures crate. There are some reasons:

• futures and futures-util are almost the same except that futures provide executors and channels. Also, both modules (executor and channel) seem to be relatively easy to make optional.
• Most of the compile time for futures seems due to futures-util, and the compile time reduction by splitting the crate doesn't seem to work very well.
• Since futures-executor also depends on futures-util, it doesn't seem to have much compile time advantage even if used individually.
• As said in https://github.com/rust-lang/futures-rs/issues/1875#issuecomment-639895854, if futures and futures-util are different crates, it seems impossible to add optional features to futures per module. (I don't plan to add a "util" feature that enables the utility itself, but I would like to add features to enable per module in the next major version.)

futures-channel is also a "utility crate", but it's almost independent, so I'm not sure if merging it is really preferable.

[Nemo157]

Sounds good to me assuming they're feature gated.

[taiki-e]

futures-channel is also a "utility crate", but it's almost independent, so I'm not sure if merging it is really preferable.

Given that futures-channel currently depends on futures-core's internal API, it probably makes sense to merge futures-channel into futures as well. This also allows AtomicWaker to move from futures-core to futures.

assuming they're feature gated.

For now, I'm considering adding the following features:

• channel
  • enable channel module
• executor
  • enable executor module
• future-util (or just future)
  • enable utilities in future module
  • Future trait is available regardless of whether the feature is enabled.
• io
  • enable io module
• lock
  • enable lock module
• sink
  • enable sink module
• stream-util (or just stream)
  • enable utilities in stream module
  • Stream trait is available regardless of whether the feature is enabled.
• task-util (or just task)
  • enable utilities in task module
  • Poll, Context, Waker etc. are available regardless of whether the feature is enabled.
  • (I haven't decided how to handle the spawn trait...)

(I'll open a new issue about this later.)

[taiki-e]

To make backporting to 0.3 easier, I will block this for now.