feat!: add skip_bits option to scramble()

[WieeRd]

Addition of IP class preservation has been postponed and the class detection has been reverted.
The skip_bits feature is useful and ready to be used on its own, so this PR is now solely dedicated to that part.

---

Basically this:

// this amount of bits will be left unanonymized;
// preserving the exact same class prefix from the original addr
let skip_bits = match addr.octets()[0] >> 4 {
    0b0000..=0b0111 => 1, // class A: 0b0...
    0b1000..=0b1011 => 2, // class B: 0b10..
    0b1100..=0b1101 => 3, // class C: 0b110.
    0b1110..=0b1110 => 4, // class D: 0b1110
    _ => return addr, // class E: 0b1111 - do not anonymize
};

Based on this (cryptopANT.c)

#define MAX_CLASS_BITS		4
	static int _class_bits[1<<MAX_CLASS_BITS] = {
		1,1,1,1,1,1,1,1, /* class A: preserve 1 bit  */
		2,2,2,2,	 /* class B: preserve 2 bits */
		3,3,		 /* class C: preserve 3 bits */
		4,		 /* class D: preserve 4 bits */
		32 		 /* class bad, preserve all  */
	};

In detail:

• Encryption loop is now stateless and ready to be parallelized
• scramble(bytes, n_bits) -> scramble(bytes, skip_bits, max_bits).
• scramble_ipv4() auto detects IP class and preserves its prefix.
• test_scramble_ipv4_full() in OpenSSL backend module has been commented out.

Unresolved Tasks/Questions:

• Is IP class still a thing? Do we really need this??
• Should I make this behavior optional? New parameter or feature flag?
• Ipv4 test dataset is now obsolete because of this change.
  • cryptopANT have their own test dataset and known to have class preserving.
  • https://github.com/DNS-OARC/cryptopANT/tree/develop/test

[SkuldNorniern]

LGTM