Add `append` methods #366

Job	Run time
miri	3m 42s
Check MSRV and minimal-versions	41s
clippy	20s
nostd_build (1.63.0, thumbv6m-none-eabi)	20s
nostd_build (stable, thumbv6m-none-eabi)	25s
tests (1.63.0)	54s
tests (stable, arbitrary)	38s
tests (stable, quickcheck)	42s
tests (stable, rayon)	53s
tests (stable, rustc-rayon)	45s
tests (stable, serde)	47s
tests (stable, borsh)	39s
tests (stable, std)	39s
tests (beta)	39s
tests (nightly, test build benchmarks)	37s
Success	3s
	12m 44s

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