Removed other instances where std Error was implemented instead of core Error

Author: LucaCappelletti94

src/parser/mod.rs

@@ -204,8 +204,7 @@ impl fmt::Display for ParserError {
     }
 }
 
-#[cfg(feature = "std")]
-impl std::error::Error for ParserError {}
+impl core::error::Error for ParserError {}
 
 // By default, allow expressions up to this deep before erroring
 const DEFAULT_REMAINING_DEPTH: usize = 50;

src/tokenizer.rs

@@ -2553,9 +2553,8 @@ mod tests {
             message: "test".into(),
             location: Location { line: 1, column: 1 },
         };
-        #[cfg(feature = "std")]
         {
-            use std::error::Error;
+            use core::error::Error;
             assert!(err.source().is_none());
         }
         assert_eq!(err.to_string(), "test at Line: 1, Column: 1");