[tz] Proposal: Sections for different kinds of backward links

[Stephen Colebourne]

I have taken a stab at splitting the `backward` file into sections.
This is based on repeated observations made by multiple people that
the file represents different kinds of Link.

https://github.com/eggert/tz/pull/33

The sections I'm suggesting are:
CROSS-ISO - These link across an ISO-3166 boundary
EXCESS - These were added despite the locations having the same
timekeeping since 1970
RENAMED - These are simple aliases/renames
OBSOLETE - These are the most obsolete form of IDs
FIXED - These are alternate names for fixed offsets

I don't expect this PR to be merged as is - There will probably need
to be some consideration as to the correct split, and as to whether
each ID is in the correct section.

The approach I've used is commented sections with stable section
names. Alternate approaches to this problem are possible. For example,
a new file could be created for each section. That seems to be more
disruptive than the proposed approach.

Note that in order for this change to be useful, the section names
need to be considered part of the source file API, and maintained over
time.

thanks
Stephen