Updating Folly Formatting's use of separators for Decimal Integers 'd' and Numbers 'n'

Updating Folly Formatting's use of separators for Decimal Integers 'd' and Numbers 'n'

Summary:
Updating Folly Formatting's use of separators for Decimal Integers 'd' and Numbers 'n'.
Updated Decimal Integers 'd' use of separators to directly use commas ',' and grouping of 3 digits.
Updated Numbers use of separators and grouping to follow locale settings.

Test Plan:
Wrote unit tests to validate the insertion of thousandsSeparators works.
Note that the numbers unit test does not modify the locale since the test cases are not thread safe.

Reviewed By: lesha@fb.com

Subscribers: tudorb, trunkagent, folly-diffs@, yfeldblum, aric

FB internal diff: D1821156

Tasks: 6087521

Signature: t1:1821156:1424923837:d0fb383a07fd733375b72b1905e6112afa141265