Added `Flock` object for automatic unlock on drop. #121

Job	Run time
linux_native_builds (x86_64-unknown-linux-gnu)	37s
rust_stable	35s
minver	12s
macos	1m 32s
linux_native_builds (x86_64-unknown-linux-musl)	41s
rustfmt	13s
tier3 (x86_64-unknown-dragonfly)	37s
tier3 (x86_64-unknown-openbsd)	38s
tier3 (armv7-unknown-linux-uclibceabihf)	27s
tier3 (x86_64-unknown-haiku)	36s
cross_compiling	0s
cross	0s
redox	0s
	6m 8s

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