Update mk_comp_tab system: make it more efficient.

* R/api.R(mk_comp_tab): Updated. Removed dependency on
calc_mz_from_smiles, instead split cpd lists into knowns and unknowns,
calculate formulas first, then the mz, then rebind knowns and
unknowns.

* R/mix.R(calc_mz_from_smiles): Removed.

(smiles2form): New function. Take list of smiles and turn them into
formulas.

(calc_mz_from_formula): Updated. Account for zero-length input.