fix: implement Drop for faidx::Reader, destroying the fai handle

[tedil]

It seems the faidx::Reader had no implementation for Drop, hence not correctly handling its inner htslib faidx handle. This might fix #390.

Todo:

• add a testcase opening and subsequently dropping many faidx handles

[coveralls]

Pull Request Test Coverage Report for Build 4978036075

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 79.307%

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Files with Coverage Reduction	New Missed Lines	%
src/bam/record.rs	1	76.33%

Totals
Change from base Build 4959211191:	-0.02%
Covered Lines:	2334
Relevant Lines:	2943

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💛 - Coveralls