feat(ser): Add ser::Error trait; avoid panic when serializing Paths

The only way to safely serialize a `Path` is to use
`.to_string_lossy()`, which replaces invalid UTF-8 characters with
the U+FFFD replacement character. Unfortunately this would lose
information, so for our default implementations, it'd be better
to punt and report an error, and leave it up to the user to
decide if they want to use the lossy encoding.

Unfortunately, we had no way for `Serializer`s to require some methods
on `Serializer::Error`, so there was no way before this patch for
the `Path` implementation to generically report that it cannot encode
this value. This adds that implementation.

breaking-change

Closes #57.

serde_tests/tests/test_ser.rs

@@ -1,12 +1,13 @@
 use std::collections::BTreeMap;
 use std::path::{Path, PathBuf};
+use std::str;
 
 use num::FromPrimitive;
 use num::bigint::{BigInt, BigUint};
 use num::complex::Complex;
 use num::rational::Ratio;
 
-use token::Token;
+use token::{self, Token};
 
 //////////////////////////////////////////////////////////////////////////
 
@@ -305,3 +306,22 @@ declare_ser_tests! {
         ],
     }
 }
+
+#[test]
+fn test_cannot_serialize_paths() {
+    let path = unsafe {
+        str::from_utf8_unchecked(b"Hello \xF0\x90\x80World")
+    };
+    token::assert_ser_tokens_error(
+        &Path::new(path),
+        &[Token::Str("Hello �World")],
+        token::Error::InvalidValue("Path contains invalid UTF-8 characters".to_owned()));
+
+    let mut path_buf = PathBuf::new();
+    path_buf.push(path);
+
+    token::assert_ser_tokens_error(
+        &path_buf,
+        &[Token::Str("Hello �World")],
+        token::Error::InvalidValue("Path contains invalid UTF-8 characters".to_owned()));
+}