Adds memory information about the script's cache to INFO

Implementation notes: as INFO is "already broken", I didn't want to break it further. Instead of computing the server.lua_script dict size on every call, I'm keeping a running sum of the body's length and dict overheads.

This implementation is naive as it **does not** take into consideration dict rehashing, but that inaccuracy pays off in speed ;)

Demo time:

```bash
$ redis-cli info memory | grep "script"
used_memory_scripts:96
used_memory_scripts_human:96B
number_of_cached_scripts:0
$ redis-cli eval "" 0 ; redis-cli info memory | grep "script"
(nil)
used_memory_scripts:120
used_memory_scripts_human:120B
number_of_cached_scripts:1
$ redis-cli script flush ; redis-cli info memory | grep "script"
OK
used_memory_scripts:96
used_memory_scripts_human:96B
number_of_cached_scripts:0
$ redis-cli eval "return('Hello, Script Cache :)')" 0 ; redis-cli info memory | grep "script"
"Hello, Script Cache :)"
used_memory_scripts:152
used_memory_scripts_human:152B
number_of_cached_scripts:1
$ redis-cli eval "return redis.sha1hex(\"return('Hello, Script Cache :)')\")" 0 ; redis-cli info memory | grep "script"
"1be72729d43da5114929c1260a749073732dc822"
used_memory_scripts:232
used_memory_scripts_human:232B
number_of_cached_scripts:2
✔ 19:03:54 redis [lua_scripts-in-info-memory L ✚…⚑] $ redis-cli evalsha 1be72729d43da5114929c1260a749073732dc822 0
"Hello, Script Cache :)"
```

src/scripting.c

@@ -919,6 +919,7 @@ void scriptingInit(int setup) {
      * This is useful for replication, as we need to replicate EVALSHA
      * as EVAL, so we need to remember the associated script. */
     server.lua_scripts = dictCreate(&shaScriptObjectDictType,NULL);
+    server.lua_scripts_mem = sizeof(dict);
 
     /* Register the redis commands table and fields */
     lua_newtable(lua);
@@ -1073,6 +1074,7 @@ void scriptingInit(int setup) {
  * This function is used in order to reset the scripting environment. */
 void scriptingRelease(void) {
     dictRelease(server.lua_scripts);
+    server.lua_scripts_mem = 0;
     lua_close(server.lua);
 }
 
@@ -1207,6 +1209,7 @@ sds luaCreateFunction(client *c, lua_State *lua, robj *body) {
      * EVALSHA commands as EVAL using the original script. */
     int retval = dictAdd(server.lua_scripts,sha,body);
     serverAssertWithInfo(c ? c : server.lua_client,NULL,retval == DICT_OK);
+    server.lua_scripts_mem += sdslen(body->ptr) + sizeof(dictEntry);
     incrRefCount(body);
     return sha;
 }