Bump core library to 4.1.0, adding cellToChildPos, childPosToCell, and cellToChildrenSize

[nrabinowitz]

• Bumps the H3 core library to v4.1.0
• Adds bindings for cellToChildPos, childPosToCell, and cellToChildrenSize (the last because it's quite useful in iterating through child cells with childPosToCell)
• Adds tests and benchmarks for the new functions

[coveralls]

Pull Request Test Coverage Report for Build 3960919727

• 26 of 27 (96.3%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 99.548%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/h3core.js	26	27	96.3%

Totals
Change from base Build 3960901929:	-0.3%
Covered Lines:	549
Relevant Lines:	550

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💛 - Coveralls

[isaacbrodsky]

We may want to also expose cellToChildrenSize in h3-java

[nrabinowitz]

We may want to also expose cellToChildrenSize in h3-java

I considered renaming it to getMaxChildPos or similar, but I guess it makes sense to stick with the core library name, even though it's named more like a cellToChildren sub-utility than part of the real API