Make availability of Error impl conditional

Author: Xaeroxe

src/exactly_one_err.rs

@@ -1,5 +1,8 @@
+#[cfg(feature = "use_std")]
 use std::error::Error;
+#[cfg(feature = "use_std")]
 use std::fmt::{Debug, Display, Formatter, Result as FmtResult};
+
 use std::iter::ExactSizeIterator;
 
 use either::Either;
@@ -70,6 +73,7 @@ where
 
 impl<I> ExactSizeIterator for ExactlyOneError<I> where I: ExactSizeIterator {}
 
+#[cfg(feature = "use_std")]
 impl<I> Display for ExactlyOneError<I> 
     where I: Iterator
 {
@@ -83,6 +87,7 @@ impl<I> Display for ExactlyOneError<I>
     }
 }
 
+#[cfg(feature = "use_std")]
 impl<I> Debug for ExactlyOneError<I> 
     where I: Iterator,
           I::Item: Debug,
@@ -102,9 +107,10 @@ impl<I> Debug for ExactlyOneError<I>
     }
 }
 
+#[cfg(feature = "use_std")]
 impl<I> Error for ExactlyOneError<I>  where I: Iterator, I::Item: Debug, {}
 
-#[cfg(test)]
+#[cfg(all(test, feature = "use_std"))]
 mod tests {
     use super::*;