Upgrade jemalloc to 0.6; add jemalloc feature #541

Job	Run time
miri	3m 5s
Measure coverage	27m 59s
Basic Checks	21m 52s
Additional Builds and Concurrency Tests	14m 55s
Release builds and tests	12m 10s
sanitize (address)	19m 20s
Slow tests	19m 7s
sanitize (address, multi-threading)	19m 12s
sanitize (leak)	7m 44s
sanitize (leak, multi-threading)	9m 39s
	2h 35m 3s

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