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`StringBuffer`: replace to_string with into_string #1819

Job	Run time
Tests / vale	10s
Tests / cargo-clippy	36s
Tests / cargo-deny	32s
Tests / amd64-linux-gnu std --release	2m 16s
Tests / amd64-linux-gnu std	1m 45s
Tests / amd64-linux-musl std --release	3m 16s
Tests / amd64-mac-native compiler	2m 42s
Tests / amd64-linux-musl std	2m 36s
Tests / cargo-fmt	20s
Tests / arm64-mac-native compiler	1m 6s
Tests / arm64-mac-native std --release	1m 47s
Tests / amd64-mac-native std --release	3m 23s
Tests / amd64-freebsd-native compiler	2m 25s
Tests / arm64-mac-native std	1m 37s
Tests / amd64-mac-native std	2m 56s
Tests / amd64-linux-musl compiler	1m 43s
Tests / amd64-linux-gnu compiler	1m 1s
Tests / amd64-freebsd-native std	3m 53s
Tests / inko-fmt	53s
Documentation	0s
Standard library documentation	0s
Nightly container	0s
	34m 57s

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