Feat: BoxFuture which implements Send (#271) #629

Job	Run time
test (x86_64-unknown-linux-gnu, stable)	12s
test (x86_64-unknown-linux-gnu, nightly)	15s
test (i686-unknown-linux-gnu, stable)	7s
test (i686-unknown-linux-gnu, nightly)	15s
test (aarch64-unknown-linux-gnu, stable)	17s
test (aarch64-unknown-linux-gnu, nightly)	16s
test (armv7-unknown-linux-gnueabihf, stable)	16s
test (armv7-unknown-linux-gnueabihf, nightly)	18s
test (riscv64gc-unknown-linux-gnu, stable)	14s
test (riscv64gc-unknown-linux-gnu, nightly)	17s
test (s390x-unknown-linux-gnu, stable)	17s
test (s390x-unknown-linux-gnu, nightly)	19s
test (x86_64-apple-darwin, stable)	20s
test (x86_64-apple-darwin, nightly)	18s
test (aarch64-apple-darwin, stable)	19s
test (aarch64-apple-darwin, nightly)	17s
test (x86_64-pc-windows-gnu, stable)	48s
test (x86_64-pc-windows-gnu, nightly)	43s
test (x86_64-pc-windows-msvc, stable)	49s
test (x86_64-pc-windows-msvc, nightly)	30s
test (i686-pc-windows-gnu, stable)	1m 4s
test (i686-pc-windows-gnu, nightly)	49s
test (i686-pc-windows-msvc, stable)	48s
test (i686-pc-windows-msvc, nightly)	48s
	10m 36s

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