de-mode-ify infer and some parts of typeck

also, fix bug in the various lint passes that fn() was considered
not suitable for the default mode

src/libcore/vec.rs

@@ -17,6 +17,7 @@ export capacity;
 export len;
 export from_fn;
 export from_elem;
+export from_slice;
 export build, build_sized;
 export to_mut;
 export from_mut;
@@ -211,6 +212,11 @@ pure fn from_elem<T: copy>(n_elts: uint, t: T) -> ~[T] {
     return v;
 }
 
+/// Creates a new unique vector with the same contents as the slice
+pure fn from_slice<T: copy>(t: &[T]) -> ~[T] {
+    from_fn(t.len(), |i| t[i])
+}
+
 /**
  * Builds a vector by calling a provided function with an argument
  * function that pushes an element to the back of a vector.