Fix ambiguous call to constraints.as_ref() (#22)

If `fast-ssh` is built with an up-to-date version of `rustc`,
the compiler complains about `constraints.as_ref()` being ambiguous,
demanding the types involved in the `.constraints()` call be clarified
with a `::<&[Constraint]>` turbofish annotation:

```
Compiling fast-ssh v0.3.1 (/build/source)
error[E0283]: type annotations needed
  --> src/layout.rs:60:10
   |
60 |         .constraints(constraints.as_ref())
   |          ^^^^^^^^^^^             ------ type must be known at this point
   |          |
   |          cannot infer type of the type parameter `C` declared on the method `constraints`
   |
   = note: multiple `impl`s satisfying `Vec<Constraint>: AsRef<_>` found in the `alloc` crate:
           - impl<T, A> AsRef<Vec<T, A>> for Vec<T, A>
             where A: Allocator;
           - impl<T, A> AsRef<[T]> for Vec<T, A>
             where A: Allocator;
help: consider specifying the generic argument
   |
60 |         .constraints::<&T>(constraints.as_ref())
   |                     ++++++

For more information about this error, try `rustc --explain E0283`.
error: could not compile `fast-ssh` (bin "fast-ssh") due to previous error
```

Luckily, if we use `constraints.as_slice()` instead, the types
involved are still apparent to the compiler, and the resulting
code still remains about as readable as before.

src/layout.rs

@@ -57,7 +57,7 @@ pub fn create_layout(app: &App, frame: &mut Frame<CrosstermBackend<Stdout>>) ->
         .direction(Direction::Horizontal)
         .margin(1)
         .horizontal_margin(0)
-        .constraints(constraints.as_ref())
+        .constraints(constraints.as_slice())
         .split(base_chunk[1]);
 
     AppLayout {