Rust embedded cross

[brainstorm]

Simplify the Cross docker setup even further now that cross-rs/cross#452 got merged upstream :)

This PR removes my transitional custom-build containers in favor of rust-embedded's cross ones.

[coveralls]

Pull Request Test Coverage Report for Build 348708859

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.968%

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Totals
Change from base Build 345332252:	0.0%
Covered Lines:	10795
Relevant Lines:	11488

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💛 - Coveralls

[brainstorm]

It seems that the lack of clang/llvm on that cross container is not good for us:

https://github.com/rust-bio/rust-htslib/runs/1084350152?check_suite_focus=true#step:12:4050

OTOH it's intriguing that the same cross command (pulling the same rust-embedded docker images and all):

cross build --release --target x86_64-unknown-linux-musl

Works locally but not on GitHub CI 🤔

[pmarks]

Looks like a nice simplification to me, if you can coax the CI to work.

[brainstorm]

The efforts on the MUSL front are not so crucial nowadays given the new developments, see issue #259