Add linear and affine equivalence between two SBoxes

[NeKroFR]

Added linear and affine equivalence between two SBoxes (-eq flag).

linear_equivalence(other) finds invertible matrices $A, B$ such that $S(x) = B \cdot T(A \cdot x)$. affine_equivalence(other) adds the offsets $a, b$ such as $S(x) = B \cdot T(A \cdot x \oplus a) \oplus b$. Both return None if no such matrices exist.

This feature is useful when dealing with obfuscated SBoxes that we suspect to be a disguised version of a known one. For example an SBox extracted from a binary that looks random but might secretly be a standard SBox wrapped in extra linear layers.

Also added a showcase pair in examples: gf16_inv (the $GF(2^4)$ inverse) and mystery, an affine disguise of it. Without the tool, the two look unrelated; with -eq, the hidden structure is recovered.

❯ python main.py -eq -in examples/gf16_inv examples/mystery
examples/gf16_inv vs examples/mystery
Affine equivalent! S(x) = B·T(A·x ⊕ a) ⊕ b, with A, a, B, b:
1 0 0 0           1       0 1 1 1         0
0 1 1 1           1       0 1 0 1         0
0 1 0 1           1       0 0 1 1         1
0 0 1 1           1       1 0 0 1         1