Virtual file support (#701)

* Add support for virtual files (eg, not backed by an OS file).

Virtual files are implemented through trait objects, with a default
implementation that tries to behave like on-disk files, but entirely
backed by in-memory structures.

Co-authored-by: Dan Gohman <sunfish@mozilla.com>

crates/wasi-common/src/lib.rs

@@ -32,6 +32,8 @@ mod memory;
 pub mod old;
 mod sandboxed_tty_writer;
 mod sys;
+mod virtfs;
+pub use virtfs::{FileContents, VirtualDirEntry};
 pub mod wasi;
 pub mod wasi32;