Add mechanism for posting functions to the main loop (#2082)

* Add mechanism for posting functions to the main loop (#1064)

* Fix indentation, add note that API is not stable

tests/device/test_schedule/test_schedule.ino

@@ -0,0 +1,77 @@
+#include <BSTest.h>
+#include <test_config.h>
+#include <Schedule.h>
+
+BS_ENV_DECLARE();
+
+void setup()
+{
+    Serial.begin(115200);
+    BS_RUN(Serial);
+}
+
+
+TEST_CASE("scheduled functions are executed", "[schedule]")
+{
+    bool executed = false;
+    CHECK(schedule_function([&](){
+        executed = true;
+    }));
+    run_scheduled_functions();
+    CHECK(executed);
+}
+
+TEST_CASE("scheduled functions are executed in correct order", "[schedule]")
+{
+    int counter = 0;
+    auto fn = [&](int id) {
+        CHECK(id == counter);
+        ++counter;
+    };
+    for (int i = 0; i < 8; ++i) {
+        schedule_function(std::bind(fn, i));
+    }
+    run_scheduled_functions();
+}
+
+TEST_CASE("functions are only executed once", "[schedule]")
+{
+    int counter = 0;
+    auto fn = [&](){
+        ++counter;
+    };
+    schedule_function(fn);
+    schedule_function(fn);
+    schedule_function(fn);
+    run_scheduled_functions();
+    CHECK(counter == 3);
+    counter = 0;
+    run_scheduled_functions();
+    CHECK(counter == 0);
+}
+
+TEST_CASE("can schedule SCHEDULED_FN_MAX_COUNT functions", "[schedule]")
+{
+    int counter = 0;
+    auto fn = [&](int id) {
+        CHECK(id == counter);
+        CHECK(id < SCHEDULED_FN_MAX_COUNT);
+        ++counter;
+    };
+    int i;
+    for (i = 0; i < SCHEDULED_FN_MAX_COUNT; ++i) {
+        CHECK(schedule_function(std::bind(fn, i)));
+    }
+    CHECK(!schedule_function(std::bind(fn, i)));
+    run_scheduled_functions();
+    CHECK(counter == SCHEDULED_FN_MAX_COUNT);
+    counter = 0;
+    for (i = 0; i < SCHEDULED_FN_MAX_COUNT; ++i) {
+        CHECK(schedule_function(std::bind(fn, i)));
+    }
+    CHECK(!schedule_function(std::bind(fn, i)));
+    run_scheduled_functions();
+    CHECK(counter == SCHEDULED_FN_MAX_COUNT);
+}
+
+void loop(){}