Macro attributes to specify From and Into trait types for structs and enums (#817)

* serde macro support for type conversions through From and Into trait

* Revisions requested by dtolnay

* Additional changes requested by dtolnay

serde/src/export.rs

@@ -7,7 +7,7 @@ use std::borrow::Cow;
 use collections::borrow::Cow;
 
 pub use core::default::Default;
-pub use core::fmt;
+pub use core::{fmt, clone, convert};
 pub use core::marker::PhantomData;
 pub use core::option::Option::{self, None, Some};
 pub use core::result::Result::{self, Ok, Err};

serde/src/lib.rs

@@ -79,7 +79,8 @@ extern crate core as actual_core;
 #[cfg(feature = "std")]
 mod core {
     pub use std::{ops, hash, fmt, cmp, marker, mem, i8, i16, i32, i64, u8, u16, u32, u64, isize,
-                  usize, f32, f64, char, str, num, slice, iter, cell, default, result, option};
+                  usize, f32, f64, char, str, num, slice, iter, cell, default, result, option,
+                  clone, convert};
     #[cfg(feature = "unstable")]
     pub use actual_core::nonzero;
 }