fix: Reason about Send/Sync-ness of types and change Rcs to Arcs

[jeeeesper]

This is an attempt to make the library a bit more usable in a multi-threaded setting. I started to replace Rc<T>s with Arc<T>s where the wrapper type is supposed to be Send and/or Sync [1]. Of course, since we work with raw C-pointers and htslib in the background, it is quite the task to actually reason about thread-safety, so the assumption so far is that the C methods behave reasonable™. I need to mention that I am not very familiar with the underlying C code. So far, this PR also does not nearly solve all the problems related to thread-safety, frankly, it might introduce more. For example, I added Send/Sync implementations for some Header types, the soundness of which I have not really verified.

This all is motivated by the use of the library in my own multi-threaded program, and of course is therefore biased to what would be convenient for me (Send/Sync on Header or Reader/Writer types for example.) For getting a Send/Sync Reader/Writer, it is of course also possible to just create Wrapper types.

Maybe this will inspire some people to continue working on this, or I will do once I find the time. Maybe, this will even revive the discussion about how to solve this issue in general. I am very open to suggestions/discussions.

There are multiple issues that are mentioning this, most importantly maybe #293 .

[1] The performance penalty is, in my opinion, absolutely tolerable and greatly outweighs having to deal with segfault when doing something "simple" as using a parrallel iterator.

[coveralls]

Pull Request Test Coverage Report for Build 19161687300

Details

• 34 of 36 (94.44%) changed or added relevant lines in 7 files are covered.
• 20 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-0.02%) to 81.912%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/bam/buffer.rs	1	2	50.0%
src/bam/header.rs	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/bam/ext.rs	1	91.88%
src/bam/mod.rs	1	83.77%
src/tbx/mod.rs	1	82.24%
src/bam/buffer.rs	3	53.13%
src/bcf/record.rs	3	82.54%
src/bam/record.rs	11	75.44%

Totals
Change from base Build 18534912582:	-0.02%
Covered Lines:	2785
Relevant Lines:	3400

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💛 - Coveralls