Cancellable timers: sorted_list::remove, timer cancel, replace done flag with timer cancellation

firmware/include/timer_queue.h

@@ -12,18 +12,27 @@ inline bool operator<(const timer_entry& a, const timer_entry& b) {
     return absolute_time_diff_us(b.when, a.when) < 0;
 }
 
+using timer_handle = sorted_list<timer_entry, 16>::node*;
+
 struct timer_queue {
     sorted_list<timer_entry, 16> queue;
     alarm_id_t alarm = -1;
     volatile bool irq_pending = false;
 
-    void schedule(absolute_time_t when, std::function<void()> fn) {
-        queue.insert({when, std::move(fn)});
+    timer_handle schedule(absolute_time_t when, std::function<void()> fn) {
+        auto* n = queue.insert({when, std::move(fn)});
         arm();
+        return n;
     }
 
-    void schedule_ms(uint32_t ms, std::function<void()> fn) {
-        schedule(make_timeout_time_ms(ms), std::move(fn));
+    timer_handle schedule_ms(uint32_t ms, std::function<void()> fn) {
+        return schedule(make_timeout_time_ms(ms), std::move(fn));
+    }
+
+    bool cancel(timer_handle h) {
+        bool removed = queue.remove(h);
+        if (removed) arm();
+        return removed;
     }
 
     void run() {