polynomial folder refactor

[odraccir1]

Reorganize the polynomial headers into topic-specific subdirectories without changing their behavior.

New layout

  polynomial/
  ├── arithmetic/
  │   ├── gcd.hpp
  │   ├── half_gcd.hpp
  │   ├── polynomial_division.hpp
  │   ├── power_series.hpp
  │   └── xgcd.hpp
  ├── backends/
  │   ├── mixed_radix_backend.hpp
  │   ├── polynomial_backend.hpp
  │   └── schoolbook_backend.hpp
  ├── factorization/
  │   ├── complete_factorization.hpp
  │   ├── distinct_degree_factorization.hpp
  │   ├── equal_degree_factorization.hpp
  │   ├── kaltofen_shoup_distinct_degree_factorization.hpp
  │   ├── polynomial_factorization.hpp
  │   └── square_free_factorization.hpp
  ├── operations/
  │   ├── basic_operations.hpp
  │   ├── basis_change.hpp
  │   ├── cyclotomic.hpp
  │   ├── evaluate.hpp
  │   ├── lagrange_interpolation.hpp
  │   └── shift.hpp
  ├── quotient_ring/
  │   ├── polynomial_composition.hpp
  │   ├── polynomial_exponentiation.hpp
  │   ├── polynomial_frobenius.hpp
  │   ├── polynomial_modular_arithmetic.hpp
  │   └── polynomial_square_root.hpp
  ├── reconstruction/
  │   └── polynomial_rational_reconstruction.hpp
  ├── types/
  │   ├── polymorphic_polynomial.hpp
  │   ├── polymorphic_polynomial_dfs.hpp
  │   └── static_simd_vector.hpp
  ├── views/
  │   ├── polynomial_dfs_view.hpp
  │   └── polynomial_view.hpp
  ├── concepts.hpp
  ├── polynomial.hpp
  └── polynomial_dfs.hpp

[spaceships]

wow, that's awesome, thanks!