Immutable array based on RRB Vector

[jialunzhang-psu]

This PR modifies the tree structure based on RRB Vector: A Practical General Purpose Immutable Sequence, ICFP'15. It will support operations including removal and concat.

[coveralls]

Pull Request Test Coverage Report for Build 4060

Details

• 64 of 93 (68.82%) changed or added relevant lines in 3 files are covered.
• 111 unchanged lines in 8 files lost coverage.
• Overall coverage decreased (-2.3%) to 78.098%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
immut/array/array.mbt	9	10	90.0%
immut/array/operation.mbt	3	11	27.27%
immut/array/tree.mbt	52	72	72.22%

Files with Coverage Reduction	New Missed Lines	%
strconv/errors.mbt	1	40.0%
strconv/bool.mbt	3	0.0%
strconv/traits.mbt	5	0.0%
strconv/double.mbt	5	55.56%
strconv/string_slice.mbt	6	66.67%
strconv/int.mbt	24	35.29%
strconv/number.mbt	30	30.65%
strconv/decimal.mbt	37	48.33%

Totals
Change from base Build 4047:	-2.3%
Covered Lines:	4286
Relevant Lines:	5488

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💛 - Coveralls