ref(cache): use Map instead of object to store JSMol cache

[RamziWeslati]

Closes #27

[RamziWeslati]

• TODO: use Map.clear at the end of the loop, instead of n calls to delete

[atfera]

I thought about something like this:

class MolCache extends Map {
  set(smiles) {
    this.set(smiles, rdkit.get_mol(smiles))
  }
  
  delete(k) {
    this.get(k).delete() // delete rdkit mol
    this.delete(k)
  }
  clear() {
    this.forEach((v, k) => {
      v.delete();
      this.delete(k); 
    )}
  }
}

// MolCache
cache.set('C')
cache.clear() //

[RamziWeslati]

looks good, I'll do that