Refactoring and renaming in [lang.v] (mostly).
- Put stuff in sections in [lang.v]. - Renamed [mk_layout] into [Layout] (for uniformity). - Renamed [it_min] and [it_max] into [max_int] and [min_int]. - Some other renamings on [int_type]-related stuff. - Changed [max_int] to be the last representable integer. - Turned [in_it_range] into an instance of [ElemOf].