[TASK] Optimizations for IPv6-functions in t3lib_div

* speedup by using IPv6-support from PHP if present
* since there is IPv6Hex2Bin(), also add IPv6Bin2Hex()
* since there is normalizeIPv6(), also add compressIPv6()
* normalization also involves converting to lowercase
* extend unit-tests

Change-Id: I7c9eda4de0f2c06f06a530f1180233d592f9dfdc
Resolves: #28669
Releases: 4.6, 4.7
Reviewed-on: http://review.typo3.org/4019
Reviewed-by: Andreas Wolf
Tested-by: Andreas Wolf