Make many functions const

The point is to set up repeated components or boilerplate before their
use sites.

The majority of these make sense as `const`. However, some functions
such as those regarding state may not make sense as `const`.

web/src/widget/button.rs

@@ -113,8 +113,8 @@ impl State {
     /// Creates a new [`State`].
     ///
     /// [`State`]: struct.State.html
-    pub fn new() -> State {
-        State::default()
+    pub const fn new() -> State {
+        State
     }
 }