Implement compile_error using call site span

Mixed site is equivalent to def site for local variables, labels, and
$crate; and equivalent to call site for all other name resolution. Since
it is being used here for a macro invocation which is not one of the
above 3 def site scenarios, directly using call_site is clearer to me.

src/error.rs

@@ -273,7 +273,7 @@ impl Error {
 
 impl ErrorMessage {
     fn to_compile_error(&self) -> TokenStream {
-        let (mut start, mut end) = (Span::mixed_site(), Span::mixed_site());
+        let (mut start, mut end) = (Span::call_site(), Span::call_site());
         if let Some(range) = self.span.get() {
             start = start.located_at(range.start);
             end = end.located_at(range.end);