Add lua_getallocf API function (#1068)

This function matches the corresponding Lua 5.1-5.4 function:

[`lua_getallocf`](https://www.lua.org/manual/5.4/manual.html#lua_getallocf)
and [source](https://www.lua.org/source/5.4/lapi.c.html#lua_getallocf)

It would be useful to get/manipulate auxiliary "userdata" pointer that was originally passed to `lua_newstate`.

VM/include/lua.h

@@ -323,6 +323,8 @@ LUA_API void lua_clonefunction(lua_State* L, int idx);
 
 LUA_API void lua_cleartable(lua_State* L, int idx);
 
+LUA_API lua_Alloc lua_getallocf(lua_State* L, void** ud);
+
 /*
 ** reference system, can be used to pin objects
 */

VM/src/lapi.cpp

@@ -1432,3 +1432,11 @@ size_t lua_totalbytes(lua_State* L, int category)
     api_check(L, category < LUA_MEMORY_CATEGORIES);
     return category < 0 ? L->global->totalbytes : L->global->memcatbytes[category];
 }
+
+lua_Alloc lua_getallocf(lua_State* L, void** ud)
+{
+    lua_Alloc f = L->global->frealloc;
+    if (ud)
+        *ud = L->global->ud;
+    return f;
+}

tests/Conformance.test.cpp

@@ -1163,6 +1163,18 @@ TEST_CASE("ApiType")
     CHECK(lua_type(L, -1) == LUA_TUSERDATA);
 }
 
+TEST_CASE("AllocApi")
+{
+    int ud = 0;
+    StateRef globalState(lua_newstate(limitedRealloc, &ud), lua_close);
+    lua_State* L = globalState.get();
+
+    void* udCheck = nullptr;
+    bool allocfIsSet = lua_getallocf(L, &udCheck) == limitedRealloc;
+    CHECK(allocfIsSet);
+    CHECK(udCheck == &ud);
+}
+
 #if !LUA_USE_LONGJMP
 TEST_CASE("ExceptionObject")
 {