Fix type inference problems in tests and docs

src/doc/trpl/looping.md

@@ -123,7 +123,7 @@ We now loop forever with `loop` and use `break` to break out early.
 iteration. This will only print the odd numbers:
 
 ```{rust}
-for x in 0..10 {
+for x in 0u32..10 {
     if x % 2 == 0 { continue; }
 
     println!("{}", x);

src/doc/trpl/threads.md

@@ -179,7 +179,7 @@ for init_val in 0 .. 3 {
 }
 
 let result = rx.recv().unwrap() + rx.recv().unwrap() + rx.recv().unwrap();
-# fn some_expensive_computation(_i: u32) -> u32 { 42 }
+# fn some_expensive_computation(_i: i32) -> i32 { 42 }
 ```
 
 Cloning a `Sender` produces a new handle to the same channel, allowing multiple
@@ -207,7 +207,7 @@ let rxs = (0 .. 3).map(|&:init_val| {
 
 // Wait on each port, accumulating the results
 let result = rxs.iter().fold(0, |&:accum, rx| accum + rx.recv().unwrap() );
-# fn some_expensive_computation(_i: u32) -> u32 { 42 }
+# fn some_expensive_computation(_i: i32) -> i32 { 42 }
 ```
 
 ## Backgrounding computations: Futures