Add SeekBuf trait and BufCursor implementation #540

Job	Run time
stable (ubuntu-latest)	1m 25s
stable (macos-latest)	2m 23s
nightly	3m 46s
stable (windows-latest)	3m 2s
cross (i686-unknown-linux-gnu)	26s
minrust	3m 0s
cross (armv7-unknown-linux-gnueabihf)	27s
rustfmt	4s
cross (powerpc-unknown-linux-gnu)	28s
tsan	2m 58s
cross (powerpc64-unknown-linux-gnu)	26s
cross (wasm32-unknown-unknown)	16s
loom	2m 46s
miri	15m 10s
Publish Documentation	2m 27s
	39m 4s

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