bitsToNat is injective: distinct bit-vectors give distinct natural numbers.
theorem
Elligator.Elligator1.σ_injective
{F : Type u_1}
[Field F]
[Fintype F]
{q : ℕ}
(field_cardinality : Fintype.card F = q)
(q_prime : Prime q)
(q_mod_4_congruent_3 : q % 4 = 3)
: