Add Vector::firstElemetn #454

Job	Run time
rust_ci (macos-latest, stable)	48s
rust_ci (macos-latest, beta)	1m 24s
rust_ci (macos-latest, nightly)	2m 20s
rust_ci (ubuntu-latest, stable)	59s
rust_ci (ubuntu-latest, beta)	1m 36s
rust_ci (ubuntu-latest, nightly)	1m 29s
rust_ci (windows-latest, stable)	1m 44s
rust_ci (windows-latest, beta)	2m 42s
rust_ci (windows-latest, nightly)	2m 46s
	15m 48s

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