mem: implement most of StandardAllocator (#860)

This was tested using `https://github.com/node-dot-cpp/alloc-test` plus a few other by-hand tests.

It seems to work for the case we care about (sysmodules without thread cache-ing).

External users are advised to build with assertions on and contact SciresM if you find issues.

This is a lot of code to have gotten right in one go, and it was written mostly after midnight while sick, so there are probably un-noticed issues.

stratosphere/dmnt/source/dmnt_main.cpp

@@ -139,7 +139,7 @@ int main(int argc, char **argv)
 
         /* Initialize threads. */
         if constexpr (NumExtraThreads > 0) {
-            const u32 priority = os::GetCurrentThreadPriority();
+            const s32 priority = os::GetCurrentThreadPriority();
             for (size_t i = 0; i < NumExtraThreads; i++) {
                 R_ABORT_UNLESS(g_extra_threads[i].Initialize(LoopServerThread, nullptr, g_extra_thread_stacks[i], ThreadStackSize, priority));
             }