(check): Check that eraseable args are indeed erasable (bugit#4)

* src/builtin.ml (level[012], type[012], type_omega, type_level)
(type_float, type_int, type_string): Move to debruijn.ml.

* src/debruijn.ml (set): New type.
(set_empty, set_mem, set_set, set_sink): New functions for it.

* src/opslexp.ml (dbset_push): New function.
(check): Check that eraseable args are indeed erasable.
Check proper types more systematically.