Fixed compareStringObject() and introduced collateStringObject().

compareStringObject was not always giving the same result when comparing
two exact strings, but encoded as integers or as sds strings, since it
switched to strcmp() when at least one of the strings were not sds
encoded.

For instance the two strings "123" and "123\x00456", where the first
string was integer encoded, would result into the old implementation of
compareStringObject() to return 0 as if the strings were equal, while
instead the second string is "greater" than the first in a binary
comparison.

The same compasion, but with "123" encoded as sds string, would instead
return a value < 0, as it is correct. It is not impossible that the
above caused some obscure bug, since the comparison was not always
deterministic, and compareStringObject() is used in the implementation
of skiplists, hash tables, and so forth.

At the same time, collateStringObject() was introduced by this commit, so
that can be used by SORT command to return sorted strings usign
collation instead of binary comparison. See next commit.

src/redis.h

@@ -995,6 +995,7 @@ int getLongDoubleFromObject(robj *o, long double *target);
 int getLongDoubleFromObjectOrReply(redisClient *c, robj *o, long double *target, const char *msg);
 char *strEncoding(int encoding);
 int compareStringObjects(robj *a, robj *b);
+int collateStringObjects(robj *a, robj *b);
 int equalStringObjects(robj *a, robj *b);
 unsigned long estimateObjectIdleTime(robj *o);