Allow enabling serde/std without also requiring serde_cbor/std to be enabled

.travis.yml

@@ -26,3 +26,4 @@ script:
     || cargo build --no-default-features --features alloc
   - cargo build --features unsealed_read_write # The crate should still build when the unsealed_read_write feature is enabled.
   - cargo build --no-default-features --features unsealed_read_write # The crate should still build when the unsealed_read_write feature is enabled and std disabled.
+  - cargo build --no-default-features --features serde/std # The crate should still build when serde/std is enabled and std disabled.

src/error.rs

@@ -4,8 +4,6 @@ use core::result;
 use serde::de;
 use serde::ser;
 #[cfg(feature = "std")]
-use std::error;
-#[cfg(feature = "std")]
 use std::io;
 
 /// This type represents all possible errors that can occur when serializing or deserializing CBOR
@@ -192,9 +190,9 @@ impl Error {
     }
 }
 
-#[cfg(feature = "std")]
-impl error::Error for Error {
-    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
+impl ser::StdError for Error {
+    #[cfg(feature = "std")]
+    fn source(&self) -> Option<&(dyn ser::StdError + 'static)> {
         match self.0.code {
             ErrorCode::Io(ref err) => Some(err),
             _ => None,