🕳 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/virtfs.rs

@@ -11,6 +11,8 @@ use std::io::SeekFrom;
 use std::path::{Path, PathBuf};
 use std::rc::Rc;
 
+pub mod pipe;
+
 /// An entry in a virtual filesystem
 pub enum VirtualDirEntry {
     /// The contents of a child directory