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.

libraries/libstratosphere/include/stratosphere/os/os_memory_common.hpp

@@ -22,4 +22,12 @@ namespace ams::os {
 
     constexpr inline size_t MemoryBlockUnitSize = 0x200000;
 
+    enum MemoryPermission {
+        MemoryPermission_None      = (0 << 0),
+        MemoryPermission_ReadOnly  = (1 << 0),
+        MemoryPermission_WriteOnly = (1 << 1),
+
+        MemoryPermission_ReadWrite = MemoryPermission_ReadOnly | MemoryPermission_WriteOnly,
+    };
+
 }