Add io::writer helper methods

It might not be immediately obvious to everyone how easy it is to use
Askama template with std::io (e.g. files) instead of std::fmt, so this
PR adds a few helper methods to make this more obvious to novice users.

askama_shared/src/lib.rs

@@ -37,9 +37,15 @@ pub trait Template: fmt::Display {
         Ok(buf)
     }
 
-    /// Renders the template to the given `writer` buffer
+    /// Renders the template to the given `writer` fmt buffer
     fn render_into(&self, writer: &mut (impl std::fmt::Write + ?Sized)) -> Result<()>;
 
+    /// Renders the template to the given `writer` io buffer
+    #[inline]
+    fn write_into(&self, writer: &mut (impl std::io::Write + ?Sized)) -> std::io::Result<()> {
+        writer.write_fmt(format_args!("{}", self))
+    }
+
     /// The template's extension, if provided
     const EXTENSION: Option<&'static str>;
 
@@ -57,9 +63,12 @@ pub trait DynTemplate {
     /// Helper method which allocates a new `String` and renders into it
     fn dyn_render(&self) -> Result<String>;
 
-    /// Renders the template to the given `writer` buffer
+    /// Renders the template to the given `writer` fmt buffer
     fn dyn_render_into(&self, writer: &mut dyn std::fmt::Write) -> Result<()>;
 
+    /// Renders the template to the given `writer` io buffer
+    fn dyn_write_into(&self, writer: &mut dyn std::io::Write) -> std::io::Result<()>;
+
     /// Helper function to inspect the template's extension
     fn extension(&self) -> Option<&'static str>;
 
@@ -79,6 +88,11 @@ impl<T: Template> DynTemplate for T {
         <Self as Template>::render_into(self, writer)
     }
 
+    #[inline]
+    fn dyn_write_into(&self, writer: &mut dyn std::io::Write) -> std::io::Result<()> {
+        writer.write_fmt(format_args!("{}", self))
+    }
+
     fn extension(&self) -> Option<&'static str> {
         Self::EXTENSION
     }
@@ -627,5 +641,9 @@ mod tests {
         assert_eq!(test.to_string(), "test");
 
         assert_eq!(format!("{}", test), "test");
+
+        let mut vec = Vec::new();
+        test.dyn_write_into(&mut vec).unwrap();
+        assert_eq!(vec, vec![b't', b'e', b's', b't']);
     }
 }