Add configurable fingerprints and MACCS keys

[sharifhsn]

Note: This PR is stacked on #50. Unique files: fingerprint.{h,cc,rs} modifications, src/fingerprint.rs, ro_mol.rs additions, test_fingerprint_params.rs.

Configurable fingerprints

The existing morgan_fingerprint() hardcodes radius=3, nBits=2048. Every ML pipeline needs to customize these:

let mol = ROMol::from_smiles("c1ccccc1CC").unwrap();

// ECFP4 (radius=2) at 1024 bits — common for random forests
let fp = mol.morgan_fingerprint_with_params(2, 1024);
assert_eq!(fp.len(), 1024);

// ECFP6 (radius=3) at 2048 bits — common for neural nets
let fp = mol.morgan_fingerprint_with_params(3, 2048);
assert_eq!(fp.len(), 2048);

Also adds:

• rdk_fingerprint_with_params(min_path, max_path, fp_size)
• pattern_fingerprint_with_params(fp_size)
• maccs_fingerprint() — 167-bit MACCS keys
• Fingerprint::len(), is_empty()

Bug fix: Fingerprint::new() now truncates to the actual bit count. Previously, non-power-of-64 fingerprints (like MACCS 167-bit) included padding from u64 block boundaries. This is backward-compatible for existing 2048-bit fingerprints (2048 is a multiple of 64).

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