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].