Sync our copy of the timezone library with IANA release tzcode2019b.

A large fraction of this diff is just due to upstream's somewhat
random decision to rename a bunch of internal variables and struct
fields.  However, there is an interesting new feature in zic:
it's grown a "-b slim" option that emits zone files without 32-bit
data and other backwards-compatibility hacks.  We should consider
whether we wish to enable that.