Molecule.__repr__ effectively hangs for large molecules

[mattwthompson]

Describe the bug

The implementation of repr for the Molecule class effectively hangs for large molecules because of (at least) an attempt to convert it to SMILES which waits for a returned value before doing any error handling. The Python docs for this record more or less say that it should return something that can re-create the object if passed to eval or summary info if that's not feasible. I suggest that for larger molecules (somewhere around 100-1000 atoms) there be no attempt to convert into SMILES, which isn't a particularly useful representation of macromolecules.

To Reproduce

Create a large Molecule from any source like a PDB file or external tool

In [24]: seq = "MNRHLCVWLFRHPSLNGYLQCHIQLHSHQFRQIHLDTRLQVFRQNRNCILHLLSKNWSRRYCHQDTKMLWKHKALQKYMENLSKEYQTLEQCLQHIPVNEENRRSLNRRHA
    ...: ELAPLAAIYQEIQETEQAIEELESMCKSLNKQDEKQLQELALEERQTIDQKINMLYNELFQSLVPKEKYDKNDVILEVTAGRTTGGDICQQFTREIFDMYQNYSCYKHWQFELLNYTP
    ...: ADYGGLHHAAARISGDGVYKHLKYEGGIHRVQRIPEVGLSSRMQRIHTGTMSVIVLPQPDEVDVKLDPKDLRIDTFRAKGAGGQHVNKTDSAVRLVHIPTGLVVECQQERSQIKNKEI
    ...: AFRVLRARLYQQIIEKDKRQQQSARKLQVGTRAQSERIRTYNFTQDRVSDHRIAYEVRDIKEFLCGGKGLDQLIQRLLQSADEEAIAELLDEHLKSAK"

In [25]: Molecule.from_rdkit(Chem.MolFromSequence(seq), allow_undefined_stereo=True)

Output

Basically stalls; if I didn't know this codebase better I'd be confused and/or think that the molecule loading failed.