Remove `#![warn(rust_2018_idioms)` (#121) #253

Job	Run time
doc	18s
clippy	14s
careful	51s
build (1.85.0, armv7a-none-eabi)	19s
miri (x86_64-unknown-linux-gnu)	41s
build (1.85.0, thumbv7em-none-eabi)	21s
rustfmt	14s
test (1.85.0)	22s
build (1.85.0, wasm32-unknown-unknown)	17s
miri (s390x-unknown-linux-gnu)	48s
test (stable)	23s
build (stable, armv7a-none-eabi)	18s
build (stable, thumbv7em-none-eabi)	18s
build (stable, wasm32-unknown-unknown)	22s
	5m 46s

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