Perf: Use computed offset passed in DateTime constructor (#1576)

* Add benchmark for DateTime.local with zone

* Use computed offset passed in DateTime constructor

All calls to the datetime constructor that pass an offset (o), have just
computed that offset (quickDT, fromObject) except for clone(). Clone
passes an "old" option to determine whether previous offset can be
reused. If we have config.o, but config.old is not set, then we can use
o without computing it.

This saves an expensive call to zone.offset that duplicates work that
was done immediately prior.

benchmarks/datetime.js

@@ -11,7 +11,7 @@ function runDateTimeSuite() {
     const formatParser = DateTime.buildFormatParser("yyyy/MM/dd HH:mm:ss.SSS");
 
     suite
-      .add("DateTime.local", () => {
+      .add("DateTime.now", () => {
         DateTime.now();
       })
       .add("DateTime.fromObject with locale", () => {
@@ -20,6 +20,9 @@ function runDateTimeSuite() {
       .add("DateTime.local with numbers", () => {
         DateTime.local(2017, 5, 15);
       })
+      .add("DateTime.local with numbers and zone", () => {
+        DateTime.local(2017, 5, 15, 11, 7, 35, { zone: "America/New_York" });
+      })
       .add("DateTime.fromISO", () => {
         DateTime.fromISO("1982-05-25T09:10:11.445Z");
       })

src/datetime.js

@@ -492,7 +492,9 @@ export default class DateTime {
       if (unchanged) {
         [c, o] = [config.old.c, config.old.o];
       } else {
-        const ot = zone.offset(this.ts);
+        // If an offset has been passed and we have not been called from
+        // clone(), we can trust it and avoid the offset calculation.
+        const ot = isNumber(config.o) && !config.old ? config.o : zone.offset(this.ts);
         c = tsToObj(this.ts, ot);
         invalid = Number.isNaN(c.year) ? new Invalid("invalid input") : null;
         c = invalid ? null : c;