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/libstratosphere/source/updater/updater_paths.cpp

@@ -34,9 +34,7 @@ namespace sts::updater {
         constexpr const char *Package2PathA = "bip:/a/package2";
 
         const char *ChooseCandidatePath(const char * const *candidates, size_t num_candidates) {
-            if (num_candidates == 0) {
-                std::abort();
-            }
+            STS_ASSERT(num_candidates > 0);
 
             for (size_t i = 0; i < num_candidates; i++) {
                 struct stat buf;