fix!: make the inner header pointer accessible again, although only though unsafe

[jeeeesper]

See #502. A recent change prematurely changed visibility of some fields from pub to pub(crate), which was not desired. This change re-exposes these fields again, albeit not directly through field access but through an unsafe method. This is one way to solve the problem, the alternative would be to make the field public again.

[coveralls]

Pull Request Test Coverage Report for Build 20429550357

Details

• 9 of 13 (69.23%) changed or added relevant lines in 2 files are covered.
• 66 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.09%) to 81.778%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/bcf/header.rs	1	5	20.0%

Files with Coverage Reduction	New Missed Lines	%
src/bam/mod.rs	1	83.77%
src/bam/header.rs	2	81.36%
src/bam/ext.rs	3	91.88%
src/bcf/mod.rs	3	78.37%
src/bam/buffer.rs	4	53.85%
src/bcf/record.rs	4	82.54%
src/bcf/buffer.rs	5	62.32%
src/bam/record_serde.rs	12	82.12%
src/bam/record.rs	32	75.6%

Totals
Change from base Build 20368714287:	-0.09%
Covered Lines:	2787
Relevant Lines:	3408

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💛 - Coveralls