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/host.rs

@@ -22,9 +22,9 @@ pub(crate) unsafe fn iovec_to_host_mut(iovec: &mut __wasi_iovec_t) -> io::IoSlic
 }
 
 #[allow(dead_code)] // trouble with sockets
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, PartialEq)]
 #[repr(u8)]
-pub(crate) enum FileType {
+pub enum FileType {
     Unknown = __WASI_FILETYPE_UNKNOWN,
     BlockDevice = __WASI_FILETYPE_BLOCK_DEVICE,
     CharacterDevice = __WASI_FILETYPE_CHARACTER_DEVICE,
@@ -39,10 +39,25 @@ impl FileType {
     pub(crate) fn to_wasi(&self) -> __wasi_filetype_t {
         *self as __wasi_filetype_t
     }
+
+    pub(crate) fn from_wasi(wasi_filetype: u8) -> Option<Self> {
+        use FileType::*;
+        match wasi_filetype {
+            __WASI_FILETYPE_UNKNOWN => Some(Unknown),
+            __WASI_FILETYPE_BLOCK_DEVICE => Some(BlockDevice),
+            __WASI_FILETYPE_CHARACTER_DEVICE => Some(CharacterDevice),
+            __WASI_FILETYPE_DIRECTORY => Some(Directory),
+            __WASI_FILETYPE_REGULAR_FILE => Some(RegularFile),
+            __WASI_FILETYPE_SOCKET_DGRAM => Some(SocketDgram),
+            __WASI_FILETYPE_SOCKET_STREAM => Some(SocketStream),
+            __WASI_FILETYPE_SYMBOLIC_LINK => Some(Symlink),
+            _ => None,
+        }
+    }
 }
 
 #[derive(Debug, Clone)]
-pub(crate) struct Dirent {
+pub struct Dirent {
     pub name: String,
     pub ftype: FileType,
     pub ino: u64,