Optimize merge

[pguillory]

Rewrote the merge operations for bitstring and array types to do fewer passes
over the data and generate less intermediate garbage. The existing benchmark
shows about a 5x improvement for Hypex.merge on arrays and 2.5x on bitstrings.

Before:

Name                                    ips        average  deviation         median         99th %
Array Hypex.merge/1                   56.60       17.67 ms    ±12.39%       17.62 ms       24.13 ms
Bitstring Hypex.merge/1               32.72       30.56 ms     ±5.22%       30.14 ms       37.23 ms

After:

Name                                    ips        average  deviation         median         99th %
Array Hypex.merge/1                  269.99        3.70 ms    ±15.78%        3.46 ms        5.71 ms
Bitstring Hypex.merge/1               78.52       12.73 ms    ±38.13%       16.16 ms       23.30 ms

[coveralls]

Pull Request Test Coverage Report for Build 19

• 7 of 7 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 15:	0.0%
Covered Lines:	53
Relevant Lines:	53

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💛 - Coveralls