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/ams_mitm/source/fs_mitm/fs_path_utils.cpp

@@ -261,9 +261,7 @@ Result FsPathUtils::Normalize(char *out, size_t max_out_size, const char *src, s
     /* Assert normalized. */
     bool normalized = false;
     R_ASSERT(FsPathUtils::IsNormalized(&normalized, out));
-    if (!normalized) {
-        std::abort();
-    }
+    STS_ASSERT(normalized);
 
     return ResultSuccess;
 }