Add initial support for a new formatting syntax

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

src/libstd/rt/io/mem.rs

@@ -26,7 +26,7 @@ pub struct MemWriter {
 }
 
 impl MemWriter {
-    pub fn new() -> MemWriter { MemWriter { buf: ~[] } }
+    pub fn new() -> MemWriter { MemWriter { buf: vec::with_capacity(128) } }
 }
 
 impl Writer for MemWriter {