mathlib

983c737a - feat(number_theory/legendre_symbol/add_character): add file introducing additive characters (#15499)

feat(number_theory/legendre_symbol/add_character): add file introducing additive characters (#15499) This adds a file that defines additive characters on commutative rings and proves some relevant properties, e.g., that on finite fields and rings of type `zmod n`, there exists a *primitive* additive character, and that the sum of the character values vanishes when the character is nontrivial. This will be used in a later PR (together with multiplicative characters) to define Gauss sums and prove some resuits on them. There is a [Zulip topic](https://leanprover.zulipchat.com/#narrow/stream/116395-maths/topic/Additive.20characters/near/290005886) to discuss this.