Add a case-insensitive option directly to siphash 2-4 implementation

Formerly, the isc_hash32() would have to change the key in a local copy to make it case insensitive. Change the isc_siphash24() and isc_halfsiphash24() functions to lowercase the input directly when reading it from the memory and converting the uint8_t * array to 64-bit (respectively 32-bit numbers).