core: replace IPAddressAccessItem with struct in_addr_prefix

core: replace IPAddressAccessItem with struct in_addr_prefix

Previously, if a unit file which contains n IPAddressAllow/Deny= lines,
then the computational order of parsing the file was O(n^3), as
ip_address_access_reduce(), whose order is O(n^2), is called for each line.

By replacing in_addr_prefix related functions, now the computational
order is O(n log n).

Fixes #20680.