Add bond access and RWMol molecule editing

[sharifhsn]

Note: This PR is stacked on #50. The diff below includes #50's changes. The unique files for this PR are: bond.{h,cc,rs}, rw_mol.{h,cc,rs}, mol_ops.{h,cc,rs} modifications, test_bond.rs, test_rw_mol_editing.rs.

Bond access

Bond<'a> follows the same Pin<&'a mut> lifetime pattern as Atom<'a>:

let mut mol = ROMol::from_smiles("c1ccccc1").unwrap();
let bond = mol.bond_with_idx(0);

assert_eq!(bond.bond_type(), BondType::AROMATIC);
assert!(bond.is_aromatic());
assert_eq!(bond.begin_atom_idx(), 0);
assert_eq!(bond.end_atom_idx(), 1);

Also exposes BondType, BondStereo, BondDir as CXX shared enums, and adds num_bonds(), bond_between_atoms() on ROMol.

RWMol editing

Build molecules programmatically:

let mut mol = RWMol::new();
mol.add_atom(6); // carbon
mol.add_atom(6);
mol.add_atom(8); // oxygen
mol.add_bond(0, 1, BondType::SINGLE);
mol.add_bond(1, 2, BondType::SINGLE);
sanitize_mol(&mut mol).unwrap();

assert_eq!(mol.as_smiles(), "CCO"); // ethanol

Also adds remove_atom(), remove_bond(), num_atoms(), Default impl, and sanitize_mol()/kekulize_mol() which are needed to validate constructed molecules.

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