Add TLSF memory usage API #311

Job	Run time
test	51s
check (thumbv6m-none-eabi, nightly)	20s
check (thumbv7em-none-eabihf, stable)	22s
check (thumbv7em-none-eabihf, nightly)	21s
check (thumbv6m-none-eabi, stable)	14s
Clippy	24s
rustdoc	17s
Format	15s
	3m 4s

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