Improve imports formatting

[Robbepop]

Introduces the following rustfmt formatting configurations:

• imports_granularity = "Crate":
  • https://github.com/rust-lang/rustfmt/blob/master/Configurations.md#crate
• imports_layout = "HorizontalVertical":
  • https://github.com/rust-lang/rustfmt/blob/master/Configurations.md#horizontalvertical

I have applied both formatting transformations in separate commits if you want to inspect their respective outcomes.
IMO it should not be a big deal to use nightly rustfmt toolchain instead of stable in our GitHub Actions CI.

I am aware that it is kinda biased to not apply standard stable Rust formatting and that introducing unstable formatting specifiers might be unpopular, however, both formatting specifiers have been "stable" for quite some time and are probably going to be stabilized as designed in the future.
Have imports merge automatically using imports_granuality instead of manually takes a lot of manual labor from developers.
The imports_layout formatting option makes Rust imports behave the same as everywhere else (e.g. in function arguments) which also conforms to a more unified formatting style.
Note: We are already using nightly Rust toolchain for clippy GitHub Actions CI.

A big advantage with vertical layouts is that Git diffs are more readable and provoke fewer merge conflicts.

There are some more nightly rustfmt options that we might want to explore using in the future in our CI, e.g.:

• https://github.com/rust-lang/rustfmt/blob/master/Configurations.md#report_fixme
• https://github.com/rust-lang/rustfmt/blob/master/Configurations.md#report_todo
• https://github.com/rust-lang/rustfmt/blob/master/Configurations.md#license_template_path

[codecov-commenter]

Codecov Report

Merging #289 (681b820) into master (dfd1eed) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #289   +/-   ##
=======================================
  Coverage   79.62%   79.62%           
=======================================
  Files          30       30           
  Lines        4756     4756           
=======================================
  Hits         3787     3787           
  Misses        969      969           

Impacted Files	Coverage Δ
src/bin/instantiate.rs	0.00% <ø> (ø)
src/func.rs	50.00% <ø> (ø)
src/global.rs	68.18% <ø> (ø)
src/host.rs	66.66% <ø> (ø)
src/imports.rs	62.71% <ø> (ø)
src/lib.rs	27.27% <ø> (ø)
src/memory/mod.rs	75.87% <ø> (ø)
src/module.rs	73.96% <ø> (ø)
src/nan_preserving_float.rs	82.81% <ø> (ø)
src/prepare/compile.rs	77.83% <ø> (ø)
... and 11 more

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