Module API for loading and saving long double

looks like each platform implements long double differently (different bit count)
so we can't save them as binary, and we also want to avoid creating a new RDB
format version, so we save these are hex strings using "%La".

This commit includes a change in the arguments of ld2string to support this.
as well as tests for coverage and short reads.

coded by @guybe7

src/networking.c

@@ -530,7 +530,7 @@ void addReplyHumanLongDouble(client *c, long double d) {
         decrRefCount(o);
     } else {
         char buf[MAX_LONG_DOUBLE_CHARS];
-        int len = ld2string(buf,sizeof(buf),d,1);
+        int len = ld2string(buf,sizeof(buf),d,LD_STR_HUMAN);
         addReplyProto(c,",",1);
         addReplyProto(c,buf,len);
         addReplyProto(c,"\r\n",2);