unimplemented encode/decode implementations for wasm and other unknowns

The cfg() guards around the encode/decode implementations result in the
crate being unusable on wasm32-unknown-unknown or any non-unix
non-windows non-redox target.

These lines mark the functionality as unimplemented on those platforms
(which is a sane behavior for platforms like wasm or bare-metal
microprocessors given those have no file system), which makes the crate
as a whole usable again for file system unrelated tasks like encoding
using the "cbor" crate.

Author: chrysn

src/serialize.rs

@@ -1371,6 +1371,10 @@ impl Encodable for path::Path {
         let v = self.as_os_str().encode_wide().collect::<Vec<_>>();
         v.encode(e)
     }
+    #[cfg(not(any(target_os = "redox", unix, windows)))]
+    fn encode<S: Encoder>(&self, _e: &mut S) -> Result<(), S::Error> {
+        unimplemented!();
+    }
 }
 
 impl Encodable for path::PathBuf {
@@ -1406,6 +1410,10 @@ impl Decodable for path::PathBuf {
         p.push(s);
         Ok(p)
     }
+    #[cfg(not(any(target_os = "redox", unix, windows)))]
+    fn decode<D: Decoder>(_d: &mut D) -> Result<path::PathBuf, D::Error> {
+        unimplemented!();
+    }
 }
 
 impl<T: Encodable + Copy> Encodable for Cell<T> {