Add proper timezone support

[darrenldl]

Need

• Code to parse tzdata database (can probably look at https://github.com/chronotope/chrono-tz as reference)
• Code to serialise said data into code file in Timere

Draft of implementation

• Given t1, t2, t3, where [t1, t2) is not DST in effect, [t2, t3) is DST in effect, then we retrieve corresponding timezone offsets offset12 for [t1, t2) and offset23 for [t2, t3)
• For an arbitrary t, we evaluate (change_tz_offset_s offset12 (interval_exc t1 t2 & t)) ||| (change_tz_offset_s offset23 (interval_exc t2 t3 & t))
  • When the "hour goes back", the intervals are still well defined
  • When the "hour goes forward", then an hour (or other duration) would be missing/not selected by above expression

[darrenldl]

Approach settled on was to parse output from zdump (see code files in gen/), and generates time_zone_data.ml in gen_artifacts. A copy stanza is added to dune files to copy gen_artifacts/time_zone_data.ml.

Right now as of 2a6a0ff , only Pattern resolution examines the transitions of time zone of interest. Hms interval is also formulated to use search based (namely it translates to between_exc).