Export std error type so no_std data formats don't need a "std" feature

[dtolnay]

This PR adds serde::ser::StdError and serde::de::StdError which are either a re-export of std::error::Error (if "std" is enabled) or a new identical trait (otherwise).

#[cfg(feature = "std")]
pub use std::error::Error as StdError;

#[cfg(not(feature = "std"))]
pub trait StdError: Debug + Display {
    fn source(&self) -> Option<&(Error + 'static)> { None }
}

Serde's error traits serde::ser::Error and serde::de::Error require std::error::Error as a supertrait, but only when Serde is built with "std" enabled. Data formats that don't care about no_std support should generally provide their error types with a std::error::Error impl directly:

#[derive(Debug)]
struct MySerError {...}

impl serde::ser::Error for MySerError {...}

impl std::fmt::Display for MySerError {...}

// We don't support no_std!
impl std::error::Error for MySerError {}

Data formats that do support no_std may either have a "std" feature of their own:

[features]
std = ["serde/std"]

#[cfg(feature = "std")]
impl std::error::Error for MySerError {}

... or else provide the std Error impl unconditionally via Serde's re-export:

impl serde::ser::StdError for MySerError {}

---

^{FYI @oli-obk, @cbeck88}

[dtolnay]

It looks like a re-export like this used to exist but was lost to time. This is from 0.7.15:

serde/serde/src/error.rs

Lines 28 to 44 in 78e7488

	pub trait Error: Debug + Display {
	/// A short description of the error.
	///
	/// The description should not contain newlines or sentence-ending
	/// punctuation, to facilitate embedding in larger user-facing
	/// strings.
	fn description(&self) -> &str;
	/// The lower-level cause of this error, if any.
	fn cause(&self) -> Option<&Error> { None }
	/// Stubbed! Returns type_id of `()`
	#[doc(hidden)]
	fn type_id(&self) -> TypeId where Self: 'static {
	TypeId::of::<()>()
	}
	}

serde/serde/src/ser/mod.rs

Lines 3 to 6 in 78e7488

	#[cfg(feature = "std")]
	use std::error;
	#[cfg(not(feature = "std"))]
	use error;

serde/serde/src/ser/mod.rs

Line 17 in 78e7488

pub trait Error: Sized + error::Error {