sys crate (#583)

Fix #508.

bdwgc-alloc/src/error.rs

@@ -0,0 +1,27 @@
+use core::{
+    error,
+    fmt::{self, Display, Formatter},
+};
+
+#[derive(Debug)]
+pub struct Error {
+    description: &'static str,
+}
+
+impl Error {
+    pub const fn new(description: &'static str) -> Error {
+        Error { description }
+    }
+}
+
+impl Display for Error {
+    fn fmt(&self, formatter: &mut Formatter) -> fmt::Result {
+        write!(formatter, "{}", self.description)
+    }
+}
+
+impl error::Error for Error {
+    fn description(&self) -> &str {
+        self.description
+    }
+}