Implement `Default` for EscapedStr. #147

Job	Run time
Rustfmt	7s
clippy	18s
ci-linux (stable, thumbv6m-none-eabi)	16s
ci-linux (stable, x86_64-unknown-linux-gnu)	24s
ci-linux (1.87.0, x86_64-unknown-linux-gnu)	25s
ci-linux (nightly, x86_64-unknown-linux-gnu, true)	26s
ci-linux (stable, thumbv7m-none-eabi)	12s
	2m 8s

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