Make RationalField behave more like a general number field

[mderickx]

Currently RationalField has significantly less functionality then NumberField_relative and NumberField_absolute . This is annoying since this means that if you want to write code that works for all numberfields you sometimes need to special case QQ.

Here is the list of functions that is currently available for NumberField_absolute, but not for QQ. 'Minkowski_embedding',

'S_class_group', 'S_unit_group', 'S_units', 'absolute_different', 'absolute_field', 'absolute_generator', 'absolute_polynomial', 'absolute_polynomial_ntl', 'absolute_vector_space', 'base_field', 'change_generator', 'change_names', 'complex_conjugation', 'complex_embeddings', 'composite_fields', 'defining_polynomial', 'different', 'disc', 'elements_of_bounded_height', 'elements_of_norm', 'factor', 'fractional_ideal', 'galois_closure', 'galois_group', 'gen_embedding', 'hilbert_conductor', 'hilbert_symbol', 'ideals_of_bdd_norm', 'integral_basis', 'is_CM', 'is_galois', 'is_isomorphic', 'is_relative', 'is_totally_imaginary', 'is_totally_real', 'latex_variable_name', 'maximal_totally_real_subfield', 'narrow_class_group', 'number_of_roots_of_unity', 'optimized_representation', 'optimized_subfields', 'pari_bnf', 'pari_nf', 'pari_polynomial', 'pari_rnfnorm_data', 'pari_zk', 'polynomial', 'polynomial_ntl', 'polynomial_quotient_ring', 'polynomial_ring', 'prime_above', 'prime_factors', 'primes_above', 'primes_of_bounded_norm', 'primes_of_degree_one_iter', 'primes_of_degree_one_list', 'primitive_element', 'primitive_root_of_unity', 'real_embeddings', 'real_places', 'reduced_basis', 'reduced_gram_matrix', 'regulator', 'relative_degree', 'relative_different', 'relative_polynomial', 'relative_vector_space', 'relativize', 'roots_of_unity', 'solve_CRT', 'specified_complex_embedding', 'structure', 'subfield', 'subfields', 'trace_dual_basis', 'trace_pairing', 'uniformizer', 'unit_group', 'units', 'vector_space', 'zeta_coefficients', 'zeta_function'

[chapoton]

See #7596

[edgarcosta]

ups, I didn't mean to change the description.

[mkoeppe]

For subfield, see #30171