Add SeekBuf trait and BufCursor implementation #528

Job	Run time
loom	2m 22s
cross (i686-unknown-linux-gnu)	45s
minrust	2m 24s
cross (armv7-unknown-linux-gnueabihf)	36s
miri	8m 3s
nightly	2m 19s
cross (powerpc-unknown-linux-gnu)	41s
rustfmt	6s
cross (powerpc64-unknown-linux-gnu)	29s
stable (ubuntu-latest)	1m 11s
tsan	2m 49s
cross (wasm32-unknown-unknown)	7s
stable (macos-latest)	1m 44s
stable (windows-latest)	2m 50s
Publish Documentation	0s
	26m 26s

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