Synthesis of the glycosyl amino acids Nα-Fmoc-Ser[Ac4-β-d-Gal p-(1 → 3)-Ac2-α-d-GalN3 p]-OPfp and Nα-Fmoc-Thr[Ac4-β-d-Gal p-(1 → 3)-Ac2-α-d-GalN3 p]-OPfp and the application in the solid-phase peptide synthesis of multiply glycosylated mucin peptides with Tn and T antigenic structures