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/server.h

@@ -1227,6 +1227,7 @@ struct redisServer {
     client *lua_client;   /* The "fake client" to query Redis from Lua */
     client *lua_caller;   /* The client running EVAL right now, or NULL */
     dict *lua_scripts;         /* A dictionary of SHA1 -> Lua scripts */
+    unsigned long long lua_scripts_mem;  /* Cached scripts' memory + oh */
     mstime_t lua_time_limit;  /* Script timeout in milliseconds */
     mstime_t lua_time_start;  /* Start time of script, milliseconds time */
     int lua_write_dirty;  /* True if a write command was called during the