core: add std::error::Error as a new primitive type (#277)

## Motivation

Currently, errors are typically recorded using their `fmt::Display` or
`fmt::Debug` implementations. This doesn't give the subscriber much
control over how the error is recorded --- in particular, it cannot
decide whether to format the error using `Display` or `Debug`, and it
cannot access the error's `source` or downcast it to another error type.
The `std::error::Error` type is implemented by a majority of errors in
both the standard library and in most crates, so its use is fairly
widespread.

## Solution

This commit adds `dyn std::error::Error + 'static` as a new primitive
type. The `'static` bound is included so that the error can be downcast.

Closes: #222

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tracing-core/src/field.rs

@@ -121,11 +121,11 @@ pub struct Iter {
 /// to be printed or stored in some other data structure.
 ///
 /// The `Visit` trait provides default implementations for `record_i64`,
-/// `record_u64`, `record_bool`, and `record_str` which simply forward the
-/// recorded value to `record_debug`. Thus, `record_debug` is the only method
-/// which a `Visit` implementation *must* implement. However, visitors may
-/// override the default implementations of these functions in order to
-/// implement type-specific behavior.
+/// `record_u64`, `record_bool`, `record_str`, and `record_error`, which simply
+/// forward the recorded value to `record_debug`. Thus, `record_debug` is the
+/// only method which a `Visit` implementation *must* implement. However,
+/// visitors may override the default implementations of these functions in
+/// order to implement type-specific behavior.
 ///
 /// Additionally, when a visitor receives a value of a type it does not care
 /// about, it is free to ignore those values completely. For example, a
@@ -162,6 +162,9 @@ pub struct Iter {
 /// `examples/counters.rs`, which demonstrates a very simple metrics system
 /// implemented using `tracing`.
 ///
+/// **Note:** the `record_error` trait method is only available when the Rust
+/// standard library is present, as it requires the `std::error::Error` trait.
+///
 /// [`Value`]: trait.Value.html
 /// [recorded]: trait.Value.html#method.record
 /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
@@ -190,6 +193,15 @@ pub trait Visit {
         self.record_debug(field, &value)
     }
 
+    /// Records a type implementing `Error`.
+    ///
+    /// **Note**: this is only enabled when the Rust standard library is
+    /// present.
+    #[cfg(feature = "std")]
+    fn record_error(&mut self, field: &Field, value: &(dyn std::error::Error + 'static)) {
+        self.record_debug(field, &format_args!("{}", value))
+    }
+
     /// Visit a value implementing `fmt::Debug`.
     fn record_debug(&mut self, field: &Field, value: &dyn fmt::Debug);
 }
@@ -313,6 +325,16 @@ impl Value for str {
     }
 }
 
+#[cfg(feature = "std")]
+impl crate::sealed::Sealed for dyn std::error::Error + 'static {}
+
+#[cfg(feature = "std")]
+impl Value for dyn std::error::Error + 'static {
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        visitor.record_error(key, self)
+    }
+}
+
 impl<'a, T: ?Sized> crate::sealed::Sealed for &'a T where T: Value + crate::sealed::Sealed + 'a {}
 
 impl<'a, T: ?Sized> Value for &'a T