Leverage setHours to have a more efficient implementation of start-end-of

[aymericbouzy]

Description of the Issue and Steps to Reproduce:

let d = new Date()
d.setHours(0, 0, 0, 0)

is way more efficient than :

let d = new Date()
d.setHours(0)
d.setMinutes(0)
d.setSeconds(0)
d.setMilliseconds(0)

(about 400 times faster)

When calling startOf(unit), where unit is one of 'hour', 'day', 'week', 'month', 'year', moment should use the first method rather than the second one.

Environment:

I'm running a ReactNative app on an iOS Simulator, running iOS 10.1.

Other information that may be helpful:

• The time zone setting of the machine the code is running on : UTC + 01:00 (Paris, France)
• The time and date at which the code was run : 2016-11-23T10:56:19+01:00
• Other libraries in use (TypeScript, Immutable.js, etc) : ReactNative

console.log( (new Date()).toString())
// [Log] Wed Nov 23 2016 10:58:47 GMT+0100 (CET) (index.ios.bundle, line 788)
console.log((new Date()).toLocaleString())
// [Log] 11/23/2016, 10:58:47 AM (index.ios.bundle, line 788)
console.log( (new Date()).getTimezoneOffset())
// [Log] -60 (index.ios.bundle, line 788)
console.log( navigator.userAgent)
// [Log] undefined (index.ios.bundle, line 788)
console.log(moment.version)
// [Log] 2.14.1 (index.ios.bundle, line 788)

[aymericbouzy]

#3620

[maggiepint]

closed in favor of #3620