Fix clippy::bool_to_int_with_if warning

```
warning: boolean to int conversion using if
   --> crossbeam-channel/src/flavors/at.rs:137:9
    |
137 | /         if self.is_empty() {
138 | |             0
139 | |         } else {
140 | |             1
141 | |         }
    | |_________^ help: replace with from: `usize::from(!self.is_empty())`
    |
    = note: `!self.is_empty() as usize` or `(!self.is_empty()).into()` can also be valid options
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#bool_to_int_with_if
    = note: `#[warn(clippy::bool_to_int_with_if)]` on by default

warning: boolean to int conversion using if
   --> crossbeam-channel/src/flavors/tick.rs:108:9
    |
108 | /         if self.is_empty() {
109 | |             0
110 | |         } else {
111 | |             1
112 | |         }
    | |_________^ help: replace with from: `usize::from(!self.is_empty())`
    |
    = note: `!self.is_empty() as usize` or `(!self.is_empty()).into()` can also be valid options
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#bool_to_int_with_if
```

Author: taiki-e

crossbeam-channel/src/flavors/at.rs

@@ -134,11 +134,7 @@ impl Channel {
     /// Returns the number of messages in the channel.
     #[inline]
     pub(crate) fn len(&self) -> usize {
-        if self.is_empty() {
-            0
-        } else {
-            1
-        }
+        usize::from(!self.is_empty())
     }
 
     /// Returns the capacity of the channel.

crossbeam-channel/src/flavors/tick.rs

@@ -105,11 +105,7 @@ impl Channel {
     /// Returns the number of messages in the channel.
     #[inline]
     pub(crate) fn len(&self) -> usize {
-        if self.is_empty() {
-            0
-        } else {
-            1
-        }
+        usize::from(!self.is_empty())
     }
 
     /// Returns the capacity of the channel.