filesystem example (#2236)

crates/wasi-common/src/virtfs.rs

@@ -127,16 +127,20 @@ impl FileContents for VecFileContents {
     }
 }
 
-struct VecFileContents {
+pub struct VecFileContents {
     content: Vec<u8>,
 }
 
 impl VecFileContents {
-    fn new() -> Self {
+    pub fn new() -> Self {
         Self {
             content: Vec::new(),
         }
     }
+
+    pub fn with_content(content: Vec<u8>) -> Self {
+        Self { content }
+    }
 }
 
 /// An `InMemoryFile` is a shared handle to some underlying data. The relationship is analagous to