* The underlying `R`/`W` doesnt need to be Clone, since we just need to
clone the Arc it sits behind.
* The rights actually shouldn't be behind an Arc - those get mutated
separately in each clone.
This introduces `Handle` implementations for readable and writable pipes, backed by arbitrary `Read`
and `Write` types, respectively. In particular, this allows for easily providing, capturing, or
redirecting WASI stdio without having to resort to OS-provided file descriptors.
The implementation is based heavily on `wasi_common::virtfs::InMemoryFile`, but without inapplicable
operations like `seek` or `allocate`.
Note that these types are not 1:1 replacements for real pipes, because they do not support `poll_oneoff`.
