Write a TimeZoneProvider that can consume zoneinfo files

[Manishearth]

Using the (yet unpublished) resb crate, we should write a TimeZoneProvider that can load from zoneinfo64.res bytes.

The format appears to be (see also: https://github.com/unicode-org/icu/blob/48597a4897e8cfea3aad5be7a4e8143c852876a5/icu4c/source/tools/tzcode/tz2icu.cpp#L164):

{
    "Names": ["string"],
    "Regions": ["string ISO region codes with some `001`"],
    "Rules": { "regioncode": intvector },
    "TzVersion": "string",
    "Zones": [ integers,
    // or 
   {
     links?: intvector,
     trans: intvector,
     typeMap: binary data,
     typeOffsets: intvector, 
     transPre32?: intvector,
     transPost32?: intvector,
     finalRaw?: integer,
     finalRule?: String,
     finalYear?: integer
  }]   
}

I don't yet know how that works, but code reading this can be found in https://github.com/unicode-org/icu/blob/48597a4897e8cfea3aad5be7a4e8143c852876a5/icu4c/source/i18n/timezone.cpp#L92

Filing issue now so that this work can be tracked.

Shane mentioned @nekevss was potentially looking into this.

[Manishearth]

Kevin has some notes in https://hackmd.io/0zPtge9tRW-swmYJzwVk0A

[nekevss]

Yeah, I've been looking into this in general. Most of the work so far has been mostly with familiarizing with how V8 was calling the resource bundles and the zoneinfo64 format. (Debug view of the format can be found here.

In general, my understanding is that ICU4X has a util crate resb that we should be able to use to access the resource bundle.

As per the above notes, my general thought so far is that this implementation would live in the timezone_provider crate with the name Zoneinfo64Provider.

My next goal was going to be digging into resb and starting to work towards an implementation. But I'm always open to help on this.

CC: @leftmostcat in case he has any input, as I think this came up in one of our chats or threads at some point.

[nekevss]

Also, an example of ures_open being called from V8 can be found here (although, there may be other places to look for a better understanding).

[nekevss]

Currently, planning on putting together an initial stub for this provider by EOW

[Manishearth]

Wonderful, thanks! I don't plan on getting that set up just yet but if in the process the compiled data can be improved to no longer need parsing that would be pretty cool. If not, I might work on that.

[robertbastian]

I'm working on adding some logic in unicode-org/icu4x#6726.