🕳 Add virtual pipes to wasi-common

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`.

crates/wasi-common/src/lib.rs

@@ -32,7 +32,7 @@ mod poll;
 mod sandboxed_tty_writer;
 pub mod snapshots;
 mod sys;
-mod virtfs;
+pub mod virtfs;
 pub mod wasi;
 
 pub use ctx::{WasiCtx, WasiCtxBuilder, WasiCtxBuilderError};