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.
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