message: core: add `Into` impls for `span::Current`
  This commit adds implementations of `Into<Option<Id>>`,
  `Into<Option<&'a Id>>`, and `Into<Option<&'static Metadata<'static>>>`.
  These should make using `span::Current` with functions like
  `Span::record_follows_from` in `tracing` more fluid.
diff: @@ -278,3 +278,27 @@ impl Current {
  }
  }
  }
  +
  +impl<'a> Into<Option<&'a Id>> for &'a Current {
  + fn into(self) -> Option<&'a Id> {
  + self.id()
  + }
  +}
  +
  +impl<'a> Into<Option<Id>> for &'a Current {
  + fn into(self) -> Option<Id> {
  + self.id().cloned()
  + }
  +}
  +
  +impl Into<Option<Id>> for Current {
  + fn into(self) -> Option<Id> {
  + self.id().cloned()
  + }
  +}
  +
  +impl<'a> Into<Option<&'static Metadata<'static>>> for &'a Current {
  + fn into(self) -> Option<&'static Metadata<'static>> {
  + self.metadata()
  + }
  +}
  