allow fn exprs to omit arg types

also, avoid using type variables for fn args with omitted types
unless necessary.  This will be important for bound regions in
fn types.

fixes #2093

src/libcore/vec.rs

@@ -1096,6 +1096,14 @@ impl extensions<T> for [T] {
     #[inline]
     fn map<U>(f: fn(T) -> U) -> [U] { map(self, f) }
     #[doc = "
+    Apply a function to the index and value of each element in the vector
+    and return the results
+    "]
+    fn mapi<U>(f: fn(uint, T) -> U) -> [U] {
+        let mut i = 0u;
+        self.map { |e| i += 1u; f(i - 1u, e) }
+    }
+    #[doc = "
     Apply a function to each element of a vector and return a concatenation
     of each result vector
     "]