feat: Allow custom attributes to be rendered

The traits as they are now may not be the final way to use them but at
least exposing them will make it possible to determine wheter the API is
good or bad.

src/doc.rs

@@ -41,14 +41,22 @@ where
     }
 }
 
-trait Render {
+/// Trait representing the operations necessary to render a document
+pub trait Render {
     type Error;
 
     fn write_str(&mut self, s: &str) -> Result<usize, Self::Error>;
-    fn write_str_all(&mut self, s: &str) -> Result<(), Self::Error>;
+    fn write_str_all(&mut self, mut s: &str) -> Result<(), Self::Error> {
+        while !s.is_empty() {
+            let count = self.write_str(s)?;
+            s = &s[count..];
+        }
+        Ok(())
+    }
 }
 
-struct IoWrite<W>(W);
+/// Writes to something implementing `std::io::Write`
+pub struct IoWrite<W>(pub W);
 
 impl<W> Render for IoWrite<W>
 where
@@ -65,7 +73,8 @@ where
     }
 }
 
-struct FmtWrite<W>(W);
+/// Writes to something implementing `std::fmt::Write`
+pub struct FmtWrite<W>(pub W);
 
 impl<W> Render for FmtWrite<W>
 where
@@ -82,7 +91,8 @@ where
     }
 }
 
-trait RenderAnnotated<A>: Render {
+/// Trait representing the operations necessary to write an annotated document.
+pub trait RenderAnnotated<A>: Render {
     fn push_annotation(&mut self, annotation: &A) -> Result<(), Self::Error>;
     fn pop_annotation(&mut self) -> Result<(), Self::Error>;
 }
@@ -177,7 +187,7 @@ impl<'a, B, A> Doc<'a, B, A> {
         B: Deref<Target = Doc<'b, B, A>>,
         W: ?Sized + io::Write,
     {
-        best(self, width, &mut IoWrite(out))
+        self.render_raw(width, &mut IoWrite(out))
     }
 
     /// Writes a rendered document to a `std::fmt::Write` object.
@@ -187,7 +197,17 @@ impl<'a, B, A> Doc<'a, B, A> {
         B: Deref<Target = Doc<'b, B, A>>,
         W: ?Sized + fmt::Write,
     {
-        best(self, width, &mut FmtWrite(out))
+        self.render_raw(width, &mut FmtWrite(out))
+    }
+
+    /// Writes a rendered document to a `RenderAnnotated<A>` object.
+    #[inline]
+    pub fn render_raw<'b, W>(&'b self, width: usize, out: &mut W) -> Result<(), W::Error>
+    where
+        B: Deref<Target = Doc<'b, B, A>>,
+        W: ?Sized + RenderAnnotated<A>,
+    {
+        best(self, width, out)
     }
 
     /// Returns a value which implements `std::fmt::Display`
@@ -212,11 +232,11 @@ impl<'a, B, A> Doc<'a, B, A> {
 }
 
 #[cfg(feature = "termcolor")]
-impl<'a, B> Doc<'a, ColorSpec, B> {
+impl<'a, B> Doc<'a, B, ColorSpec> {
     #[inline]
     pub fn render_colored<'b, W>(&'b self, width: usize, out: W) -> io::Result<()>
     where
-        B: Deref<Target = Doc<'b, ColorSpec, B>>,
+        B: Deref<Target = Doc<'b, B, ColorSpec>>,
         W: WriteColor,
     {
         best(