Restructure zone getter to allow for lazy unpacking.

[timrwood]

Another alternative to #211 and #215.

This approach is much closer to the approach in #211, but cleans up a bunch of internals in the process.

We load data like below. This is the example at https://jsfiddle.net/rn58wmvn/3/.

moment.tz.load({
    version: "2015a",
    zones: [ "America/Chicago|CST CDT|60 50|01010101010101010101010|1BQU0 1zb0 Op0 1zb0 Op0 1zb0 Op0 1zb0 Op0 1zb0 Op0 1zb0 Rd0 1zb0 Op0 1zb0 Op0 1zb0 Op0 1zb0 Op0 1zb0"],
    links: ["America/Chicago|US/Central"]
});

The internal representation looks like this.

var links = {
  america_chicago: 'us_central',
  us_central: 'america_chicago'
};
var zones = {
  america_chicago: 'America/Chicago|CST CDT|60 50|0101010101010101...'
};
var names = {
  america_chicago: 'America/Chicago',
  us_central: 'US/Central'
};

Now we try to load the data for US/Central.

We check if there is an unpacked zone for us_central. There is not, so we continue.

We then check if there is a packed string for us_central. There is not, so we continue.

We then check if there is a link for us_central. There is a link, america_chicago, so we call getZone on america_chicago.

We check if there is an unpacked zone for america_chicago. There is not, so we continue.

We then check if there is a packed string for america_chicago. There is, so we unpack it. The data now looks like this.

var links = {
  america_chicago: 'us_central',
  us_central: 'america_chicago'
};
var zones = {
  america_chicago: Zone(name: 'America/Chicago', abbrs: ['CST', 'CDT', '...'])
};
var names = {
  america_chicago: 'America/Chicago',
  us_central: 'US/Central'
};

That zone is returned to the function looking for the us_central zone, and we use that and the names hash to populate the new zone. The data looks like this.

var links = {
  america_chicago: 'us_central',
  us_central: 'america_chicago'
};
var zones = {
  america_chicago: Zone(name: 'America/Chicago', abbrs: ['CST', 'CDT', '...'])
  us_central: Zone(name: 'US/Central', abbrs: ['CST', 'CDT', '...'])
};
var names = {
  america_chicago: 'America/Chicago',
  us_central: 'US/Central'
};

Subsequent calls to get the zone for America/Chicago and US/Central will now be short-circuted by the check for an unpacked zone.

The perf for this approach is roughly the same for the approach in #211.

This approach. http://jsperf.com/moment-timezone-unpack-on-demand/5
#211 approach. http://jsperf.com/moment-timezone-unpack-on-demand-2/2

I've only tested on a couple desktop browsers, so it would be good to see if the same perf improvements seen on android with @davidpean's implementation are applied here as well. cc @paulirish

[davidpean]

@timrwood seeing a similar 25-50ms load time on my Android -- Nice!!

[paulirish]

Very nice. :D