lib: add no-std error support (#478)

* feat: add no-std error support
* refactor: reduce cfgs

Author: hannahfluch

src/error.rs

@@ -2,12 +2,10 @@
 //!
 
 use alloc::string::String;
+use core::error;
 use core::fmt;
 use core::num::TryFromIntError;
 use core::result;
-#[cfg(feature = "std")]
-use std::{error, io};
-
 #[non_exhaustive]
 #[derive(Debug)]
 /// A custom Goblin error
@@ -20,15 +18,15 @@ pub enum Error {
     Scroll(scroll::Error),
     /// An IO based error
     #[cfg(feature = "std")]
-    IO(io::Error),
+    IO(std::io::Error),
     /// Buffer is too short to hold N items
     BufferTooShort(usize, &'static str),
 }
 
-#[cfg(feature = "std")]
 impl error::Error for Error {
     fn source(&self) -> Option<&(dyn error::Error + 'static)> {
         match *self {
+            #[cfg(feature = "std")]
             Error::IO(ref io) => Some(io),
             Error::Scroll(ref scroll) => Some(scroll),
             _ => None,
@@ -37,8 +35,8 @@ impl error::Error for Error {
 }
 
 #[cfg(feature = "std")]
-impl From<io::Error> for Error {
-    fn from(err: io::Error) -> Error {
+impl From<std::io::Error> for Error {
+    fn from(err: std::io::Error) -> Error {
         Error::IO(err)
     }
 }