Add functions for byte characters and doc comments

Likely to be tweaked when the actual PR lands!
diff --git a/src/lib.rs b/src/lib.rs
index d7891cb..e1305eb 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -191,9 +191,17 @@
 }
 
 impl Literal {
+    pub fn bytechar(b: u8) -> Literal {
+        Literal(imp::Literal::bytechar(b))
+    }
+
     pub fn bytestring(s: &[u8]) -> Literal {
         Literal(imp::Literal::bytestring(s))
     }
+
+    pub fn doccomment(s: &str) -> Literal {
+        Literal(imp::Literal::doccomment(s))
+    }
 }
 
 macro_rules! tys {