Add an is_directory() helper method. (#1373)

This allows `ctx` to avoid depending on wasi::FileType.

crates/wasi-common/src/ctx.rs

@@ -351,7 +351,7 @@ impl WasiCtxBuilder {
                     }
                 }
                 Descriptor::VirtualFile(virt) => {
-                    if virt.get_file_type() != types::Filetype::Directory {
+                    if !virt.is_directory() {
                         return Err(WasiCtxBuilderError::NotADirectory(guest_path));
                     }
                 }

crates/wasi-common/src/virtfs.rs

@@ -137,6 +137,10 @@ pub(crate) trait VirtualFile: MovableFile {
 
     fn get_file_type(&self) -> types::Filetype;
 
+    fn is_directory(&self) -> bool {
+        self.get_file_type() == types::Filetype::Directory
+    }
+
     fn get_rights_base(&self) -> types::Rights {
         types::Rights::empty()
     }