Use standard sequent calculus with sequents

[rzach]

The sequent calculus is presented with sequents being sets of formulas. Switch to sequences so it is more standard and allows future expansion by discussion of substructural logics.

[nicolewyatt]

How time consuming will this fix be? I would like this for the Fall but I realize time is getting short.

[rzach]

Should be doable; the harder part will be to fix arithmetization of syntax for LK, but you don't need that.