Add: header_included_v6() and set_header_included_v6() #1069

Job	Run time
Check (armv7-sony-vita-newlibeabihf)	35s
Clippy	15s
Check (i686-unknown-hurd-gnu)	38s
Docs (aarch64-apple-ios)	12s
Check (aarch64-apple-ios)	17s
Docs (aarch64-linux-android)	16s
Docs (armv7-sony-vita-newlibeabihf)	28s
Check (aarch64-linux-android)	14s
Docs (x86_64-apple-darwin)	14s
Docs (i686-unknown-hurd-gnu)	26s
Check (x86_64-apple-darwin)	14s
Docs (x86_64-unknown-fuchsia)	13s
Check (x86_64-unknown-fuchsia)	19s
Docs (x86_64-pc-windows-msvc)	13s
Check (x86_64-pc-windows-msvc)	15s
Rustfmt	14s
Docs (x86_64-pc-solaris)	16s
Test (stable)	15s
Check (x86_64-pc-solaris)	22s
Test (1.63.0)	1m 28s
Docs (x86_64-unknown-freebsd)	13s
Check (x86_64-unknown-freebsd)	15s
Check (x86_64-unknown-illumos)	15s
Docs (x86_64-unknown-illumos)	15s
Test (beta)	16s
Check (x86_64-unknown-linux-gnu)	15s
Docs (x86_64-unknown-linux-gnu)	14s
Test (nightly)	13s
Check (x86_64-unknown-linux-musl)	20s
Docs (x86_64-unknown-linux-musl)	17s
Test (macos)	29s
Check (x86_64-unknown-netbsd)	13s
Docs (x86_64-unknown-netbsd)	13s
Test (windows)	44s
Check (x86_64-unknown-redox)	18s
Docs (x86_64-unknown-redox)	13s
Check (armv7-linux-androideabi)	13s
Docs (armv7-linux-androideabi)	15s
Check (i686-linux-android)	15s
Docs (i686-linux-android)	16s
	13m 6s

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