Add initial support for a new formatting syntax

The new macro is available under the name ifmt! (only an intermediate name)

src/libsyntax/syntax.rs

@@ -73,6 +73,7 @@ pub mod ext {
 
     pub mod cfg;
     pub mod fmt;
+    pub mod ifmt;
     pub mod env;
     pub mod bytes;
     pub mod concat_idents;