Add opt to use existing loc obj in weekdays/months

[dan-overton]

Potential fix for #775 .

This allows an existing loc object to be passed into Info months and weekdays functions, to be used instead of creating a new one. The use of these in DateTime has been modified to use this option.

In a loop of 100000 uses of DateTime.weekdayLong this appears to be roughly twice as fast as creating a Locale each time.

[icambron]

Good stuff. Can you add something to the benchmarks code?

[dan-overton]

Sure, will do

[dan-overton]

I've added some benchmarks. The benchmarks needed to be at the Info level to compare running with / without an existing locale object. That led to me refactoring the benchmarks themselves to run multiple files. Let me know what you think - I'm happy to roll it back a bit if there's a simpler way that works.

[icambron]

Seems fine. Thanks!