Add desugar command and desugar programs

src/span.rs

@@ -24,6 +24,14 @@ impl Span {
         let end = self.end.max(other.end);
         Self::new(start, end)
     }
+
+    pub fn at_start(self) -> Self {
+        Self::new(self.start, self.start)
+    }
+
+    pub fn at_end(self) -> Self {
+        Self::new(self.end, self.end)
+    }
 }
 
 impl fmt::Debug for Span {