Use edition 2021

[Robbepop]

Upgrade was trivial. Just a few doc tests in ink_storage had to be upgraded.

[codecov-commenter]

Codecov Report

Merging #977 (453abc6) into master (ddf40e9) will increase coverage by 3.52%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #977      +/-   ##
==========================================
+ Coverage   75.47%   79.00%   +3.52%     
==========================================
  Files         244      244              
  Lines        9147     9149       +2     
==========================================
+ Hits         6904     7228     +324     
+ Misses       2243     1921     -322     

Impacted Files	Coverage Δ
crates/storage/src/collections/vec/mod.rs	97.29% <ø> (ø)
crates/allocator/src/bump.rs	62.50% <0.00%> (-4.17%)	⬇️
crates/lang/ir/src/ir/attrs.rs	83.07% <0.00%> (+4.07%)	⬆️
crates/lang/ir/src/ir/item_impl/callable.rs	91.75% <0.00%> (+4.12%)	⬆️
crates/lang/ir/src/ir/trait_def/item/mod.rs	90.08% <0.00%> (+4.95%)	⬆️
crates/engine/src/ext.rs	61.18% <0.00%> (+6.57%)	⬆️
crates/lang/ir/src/ir/item_impl/constructor.rs	91.52% <0.00%> (+6.77%)	⬆️
crates/lang/ir/src/ir/selector.rs	88.00% <0.00%> (+8.00%)	⬆️
crates/lang/ir/src/ir/idents_lint.rs	71.42% <0.00%> (+9.52%)	⬆️
crates/lang/ir/src/ir/item_impl/message.rs	91.56% <0.00%> (+15.66%)	⬆️
... and 17 more

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