Bump MSRV to 1.42 and use subslices pattern

[natir]

This pull request increase the minimal supported rust version to 1.42, because object one of our dependencies requires it.

But I'm also taking the opportunity to use the subslices pattern which are available since version 1.42. These changes mean that we no longer use the values associated with the Format enum, so I'm removing the enum_primitive dependency.

[codecov]

Codecov Report

Merging #42 (8250009) into master (a49b0d5) will decrease coverage by 0.71%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
- Coverage   94.92%   94.21%   -0.72%     
==========================================
  Files           9        9              
  Lines         394      380      -14     
==========================================
- Hits          374      358      -16     
- Misses         20       22       +2     

Impacted Files	Coverage Δ
src/level.rs	97.22% <66.66%> (ø)
src/basic/compression.rs	100.00% <100.00%> (ø)
src/send/compression.rs	100.00% <100.00%> (ø)
src/send/mod.rs	98.75% <0.00%> (-1.25%)	⬇️
src/basic/mod.rs	98.75% <0.00%> (-1.25%)	⬇️

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[luizirber]

Nice! I added a commit to disable a clippy lint (that suggested we change an error name, which is a major version bump...), and then it triggered other 15 warnings related to returning a Result<Box<...>> when a Box<...> would suffice (since there is only one code path), but that is a false positive (because we do return an Err(FeatureDisabled) in another path of the macro...

So, I added a #![allow(clippy::unnecessary_wraps)] to src/send/compression.rs and src/basic/compression.rs.

The final clippy lint was for replacing the Into<u32> for Level and similar with From<Level> for u32 (because the Into impl comes for free if we implement the From trait).