Impl std::error::Error for VerboseError

Users of rust libraries usually expect error types to implement the
`std::error::Error` trait, and many error utilities will naturally only
work with types implementing that trait. However, unless nom's custom
`Error` type, the `VerboseError` did not implement the std trait.

This change introduces an empty implementation, meaning that users will
only benefit from `Display` and `Debug` in the end.

src/error.rs

@@ -222,6 +222,9 @@ impl<I: fmt::Display> fmt::Display for VerboseError<I> {
   }
 }
 
+#[cfg(feature = "std")]
+impl<I: fmt::Debug + fmt::Display> std::error::Error for VerboseError<I> {}
+
 use crate::internal::{Err, IResult};
 
 /// Create a new error from an input position, a static string and an existing error.