Added `Flock` object for automatic unlock on drop. #125

Job	Run time
rust_stable	37s
minver	12s
macos	57s
linux_native_builds (x86_64-unknown-linux-gnu)	37s
linux_native_builds (x86_64-unknown-linux-musl)	36s
rustfmt	14s
tier3 (x86_64-unknown-dragonfly)	37s
tier3 (x86_64-unknown-openbsd)	39s
tier3 (armv7-unknown-linux-uclibceabihf)	24s
tier3 (x86_64-unknown-haiku)	36s
redox	0s
cross_compiling	0s
cross	0s
	5m 29s

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