Remove redundant functions for Documents

There's no need for a `string` function since one already
exists for Nodes.

Now the root node is always stored on the document. This
means callers of `ts_document_root_node` don't need to release
its return value.

src/compiler/generate_code/c_code.cc

@@ -235,7 +235,7 @@ class CCodeGenerator {
       return lookahead + " == " + escape_char(range.min);
     } else {
       return escape_char(range.min) + string(" <= ") + lookahead + " && " +
-          lookahead + " <= " + escape_char(range.max);
+             lookahead + " <= " + escape_char(range.max);
     }
   }