Structure and conformation of the muscarinic agonists 3-(3-amino-1,2,4-oxadiazol-5-yl)-1-azabicyclo[2.2.2]octane and 1,2,5,6-tetrahydro-1-methyl-3-pyridinecarboxaldehyde oxime and related tertiary amine, quaternary ammonium and sulfonium analogues