Use a separate trait for object safety (#579)

This is relatively major change to the main trait's API. For context,
I always started from the concept of monomorphized traits, but later
several contributors asked about object safety. At that point I made
`Template` object-safe, and then even later added a `SizedTemplate`
to make some things easier for people who don't need object safety.

However, having object-safety in the primary trait is bad for
performance (a substantial number of calls into the virtual `Write`
trait is relatively slow), and I don't think those who don't need
object safety should pay for the cost of having it.

Additionally, I feel using associated consts for the extension and
size hint is more idiomatic than having accessor methods. I don't
know why I didn't use these from the start -- maybe associated
consts didn't exist yet, or I didn't yet know how/when to use them.
Askama is pretty old at this point...

1 files changed, 2 insertions, 5 deletions

diff --git a/testing/tests/simple.rs b/testing/tests/simple.rs
index e66485a..dcd324e 100644
--- a/testing/tests/simple.rs
+++ b/testing/tests/simple.rs
@@ -1,6 +1,6 @@
 #![allow(clippy::blacklisted_name)]
 
-use askama::{SizedTemplate, Template};
+use askama::Template;
 
 use std::collections::HashMap;
 
@@ -26,10 +26,7 @@ fn test_variables() {
          Iñtërnâtiônàlizætiøn is important\n\
          in vars too: Iñtërnâtiônàlizætiøn"
     );
-    assert_eq!(
-        <VariablesTemplate as SizedTemplate>::extension(),
-        Some("html")
-    );
+    assert_eq!(VariablesTemplate::EXTENSION, Some("html"));
 }
 
 #[derive(Template)]