Add the simplest heap #253

Job	Run time
check (thumbv7m-none-eabi, stable)	29s
check (thumbv7m-none-eabi, nightly)	22s
test	3m 31s
Clippy	19s
check (thumbv6m-none-eabi, stable)	32s
check (thumbv6m-none-eabi, nightly)	22s
rustdoc	27s
Format	14s
	6m 16s

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