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 4255

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 64 of 93 (68.82%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 81.041%

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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%

Totals
Change from base Build 4248:	-0.3%
Covered Lines:	4561
Relevant Lines:	5628

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💛 - Coveralls