misc: bump Rust to 2021 edition

[Wodann]

No description provided.

[codecov]

Codecov Report

Merging #367 (3304de1) into main (c970d80) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #367      +/-   ##
==========================================
- Coverage   82.63%   82.55%   -0.08%     
==========================================
  Files         264      264              
  Lines       15297    15291       -6     
==========================================
- Hits        12641    12624      -17     
- Misses       2656     2667      +11     

Impacted Files	Coverage Δ
crates/mun_runtime/src/lib.rs	68.81% <100.00%> (ø)
crates/mun_codegen/src/code_gen/symbols.rs	78.65% <0.00%> (-4.27%)	⬇️
crates/mun_codegen/src/ir/file.rs	84.48% <0.00%> (-3.45%)	⬇️
crates/mun_memory/src/diff.rs	80.89% <0.00%> (-1.28%)	⬇️
crates/mun_codegen/src/type_info.rs	66.15% <0.00%> (-0.52%)	⬇️
crates/mun_compiler/src/driver.rs	63.55% <0.00%> (-0.17%)	⬇️
crates/mun_hir/src/package_defs/tests.rs	85.88% <0.00%> (-0.17%)	⬇️
crates/mun_hir/src/ty.rs	78.43% <0.00%> (-0.15%)	⬇️
crates/mun_hir/src/code_model/function.rs	95.89% <0.00%> (-0.06%)	⬇️
crates/mun_hir/src/code_model/struct.rs	98.82% <0.00%> (-0.02%)	⬇️

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