Remove `#![warn(rust_2018_idioms)]` (#657) #1107

Job	Run time
minimal-versions / minimal-versions	57s
set-msrv / set-msrv	2s
cross (1.85.0, aarch64-unknown-linux-gnu, default)	36s
test (1.85.0)	55s
cross (1.85.0, powerpc-unknown-linux-gnu, default)	45s
build (1.85.0, thumbv7em-none-eabi)	20s
test (stable)	52s
cross (stable, aarch64-unknown-linux-gnu, default)	45s
build (1.85.0, wasm32-unknown-unknown)	22s
cross (stable, powerpc-unknown-linux-gnu, default)	37s
build (stable, thumbv7em-none-eabi)	24s
build (stable, wasm32-unknown-unknown)	24s
	6m 59s

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