refactor(kernel): address clippy lints

- `len_without_is_empty`
- `declare_interior_mutable_const` (ignored)
- `missing_safety_doc` (ignored)

Unaddressed:
- `redundant_pattern_matching` - This is a bug in clippy:
  <rust-lang/rust-clippy#5697>

src/constance/src/kernel.rs

@@ -81,10 +81,12 @@ pub unsafe trait KernelCfg1: Sized + 'static {
 /// implementation.
 ///
 /// These methods are only meant to be called by the kernel.
+#[allow(clippy::missing_safety_doc)]
 pub unsafe trait Port: KernelCfg1 {
     type PortTaskState: Send + Sync + Init + 'static;
 
     /// The initial value of [`TaskCb::port_task_state`] for all tasks.
+    #[allow(clippy::declare_interior_mutable_const)] // it's intentional
     const PORT_TASK_STATE_INIT: Self::PortTaskState;
 
     /// The default stack size for tasks.
@@ -148,6 +150,7 @@ pub unsafe trait Port: KernelCfg1 {
 /// # Safety
 ///
 /// These are only meant to be called by the port.
+#[allow(clippy::missing_safety_doc)]
 pub trait PortToKernel {
     /// Initialize runtime structures.
     ///

src/constance/src/kernel/cfg/vec.rs

@@ -28,6 +28,10 @@ impl<T: Copy> ComptimeVec<T> {
         self.len
     }
 
+    pub const fn is_empty(&self) -> bool {
+        self.len == 0
+    }
+
     // FIXME: Waiting for <https://github.com/rust-lang/rust/issues/67792>
     pub const fn get(&self, i: usize) -> T {
         if i >= self.len() {