break out segment.rs file

[robfitzgerald]

the file transportation_segment.rs contains TransportationSegmentRecord and all of its inner types. because of deep nesting and adding implementations and derivations to sub-types, this file is now 800 lines long. in place of transportation_segment.rs, we should have a segment module with separate files for separate types, such as

record/
  mod.rs
  ..
  segment/
    mod.rs
    record.rs
    subtype.rs
    class.rs
    subclass.rs
    access_restriction.rs
    access_restriction_when.rs
    ..

note: in some cases, the types may be better suited to be slotted into the common.rs module.