os: implement waitable management.

This implements waitable management for Events (and
implements Events). It also refactors PM to use new
Event/Waitable semantics, and also adds STS_ASSERT
as a macro for asserting a boolean expression. The
rest of stratosphere has been refactored to use
STS_ASSERT whenever possible.

stratosphere/boot/source/gpio/gpio_initial_configuration.cpp

@@ -80,9 +80,7 @@ namespace sts::gpio {
         }
 
         /* Ensure we found an appropriate config. */
-        if (configs == nullptr) {
-            std::abort();
-        }
+        STS_ASSERT(configs != nullptr);
 
         for (size_t i = 0; i < num_configs; i++) {
             /* Configure the GPIO. */