Allow for unused `pub use` in experimental API which doesn't have its mods public for noiw.
MIPS CI is fully broken (doesn't find the MIPS toolchain) so disable it for now.
Reenable powerpc64 which is no longer broken
These are the stream equivalents to `std::iter::Iterator::sum()` and
`std::iter::Iterator::product()`.
Note that this changeset tweaks the `Stream::Sum` and `Stream::Product`
traits a little: rather than returning a generic future `F`, they return
a pinned, boxed, `Future` trait object now. This is in line with other
traits that return a future, e.g. `FromStream`.
