Add ID_onehot and ID_onehot0 expressions

[danpoe]

ID_onehot and ID_onehot0 expressions are used in cbmc/develop but there are no corresponding expression types in std_expr.h. Moreover, ID_onehot and ID_onehot0 are not assigned in cbmc/develop (they are assigned in EBMC).

New expression types should be added to std_expr.h and then the code in cbmc/develop should be refactored to use those instead of plain exprt.