Add ring info queries and 20 targeted molecular descriptors

[sharifhsn]

Note: This PR is stacked on #50. Unique files: ring_info.{h,cc,rs}, descriptors.{h,cc,rs} modifications, src/descriptors.rs additions, test_ring_info.rs, test_extended_descriptors.rs.

Ring info

Direct ring queries on ROMol without a separate lifetime-bound struct:

let mol = ROMol::from_smiles("c1ccc2ccccc2c1").unwrap(); // naphthalene

assert_eq!(mol.num_rings(), 2);
assert!(mol.is_atom_in_ring_of_size(0, 6));
assert_eq!(mol.atom_ring_sizes(0), vec![6]);

20 targeted descriptors

Direct method calls on ROMol instead of Properties::compute_properties():

let mol = ROMol::from_smiles("c1ccccc1").unwrap();

// Lipinski filter in 4 calls — no HashMap, no computing 36 unused descriptors
let passes = mol.calc_amw() <= 500.0
    && mol.calc_clog_p() <= 5.0
    && mol.calc_num_hbd() <= 5
    && mol.calc_num_hba() <= 10;

Full list: calc_exact_mw, calc_amw, calc_mol_formula, calc_num_heavy_atoms, calc_fraction_csp3, calc_labute_asa, calc_tpsa, calc_clog_p, calc_num_hbd, calc_num_hba, calc_num_rotatable_bonds, calc_num_amide_bonds, calc_num_heteroatoms, calc_num_aromatic_rings, calc_num_aliphatic_rings, calc_num_saturated_rings, calc_num_heterocycles, calc_num_aromatic_heterocycles, calc_num_spiro_atoms, calc_num_bridgehead_atoms.

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