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;
 }

src/server.c

@@ -2994,6 +2994,7 @@ sds genRedisInfoString(char *section) {
         char peak_hmem[64];
         char total_system_hmem[64];
         char used_memory_lua_hmem[64];
+        char used_memory_scripts_hmem[64];
         char used_memory_rss_hmem[64];
         char maxmemory_hmem[64];
         size_t zmalloc_used = zmalloc_used_memory();
@@ -3013,6 +3014,7 @@ sds genRedisInfoString(char *section) {
         bytesToHuman(peak_hmem,server.stat_peak_memory);
         bytesToHuman(total_system_hmem,total_system_mem);
         bytesToHuman(used_memory_lua_hmem,memory_lua);
+        bytesToHuman(used_memory_scripts_hmem,server.lua_scripts_mem);
         bytesToHuman(used_memory_rss_hmem,server.cron_malloc_stats.process_rss);
         bytesToHuman(maxmemory_hmem,server.maxmemory);
 
@@ -3037,6 +3039,9 @@ sds genRedisInfoString(char *section) {
             "total_system_memory_human:%s\r\n"
             "used_memory_lua:%lld\r\n"
             "used_memory_lua_human:%s\r\n"
+            "used_memory_scripts:%lld\r\n"
+            "used_memory_scripts_human:%s\r\n"
+            "number_of_cached_scripts:%lu\r\n"
             "maxmemory:%lld\r\n"
             "maxmemory_human:%s\r\n"
             "maxmemory_policy:%s\r\n"
@@ -3069,6 +3074,9 @@ sds genRedisInfoString(char *section) {
             total_system_hmem,
             memory_lua,
             used_memory_lua_hmem,
+            server.lua_scripts_mem,
+            used_memory_scripts_hmem,
+            dictSize(server.lua_scripts),
             server.maxmemory,
             maxmemory_hmem,
             evict_policy,

src/server.h

@@ -1203,6 +1203,7 @@ 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