mem: implement most of StandardAllocator

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/ams_mitm/source/fs_mitm/fsmitm_module.cpp

@@ -49,7 +49,7 @@ namespace ams::mitm::fs {
         void ProcessForServerOnAllThreads() {
             /* 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));
                 }