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/test-programs/wasi-tests/src/bin/fd_flags_set.rs

@@ -50,7 +50,7 @@ unsafe fn test_fd_fdstat_set_flags(dir_fd: wasi::Fd) {
         )
         .expect("reading file"),
         buffer.len(),
-        "shoudl read {} bytes",
+        "should read {} bytes",
         buffer.len()
     );
 
@@ -87,7 +87,7 @@ unsafe fn test_fd_fdstat_set_flags(dir_fd: wasi::Fd) {
         )
         .expect("reading file"),
         buffer.len(),
-        "shoudl read {} bytes",
+        "should read {} bytes",
         buffer.len()
     );
 
@@ -126,7 +126,7 @@ unsafe fn test_fd_fdstat_set_flags(dir_fd: wasi::Fd) {
         )
         .expect("reading file"),
         buffer.len(),
-        "shoudl read {} bytes",
+        "should read {} bytes",
         buffer.len()
     );