Create a replacement for Cell<SM primitive>. Fixes #4337.

[jdm]

We still will need a lint to ban any further uses, but that can be a followup.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3524

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[jdm]

r? @Ms2ger

[Manishearth]

Needs rebase

[jdm]

r=Manishearth

[bors-servo]

saw approval from Manishearth
at jdm@8ff3e6b

[bors-servo]

merging jdm/servo/heapsm2 = 8ff3e6b into auto

[bors-servo]

jdm/servo/heapsm2 = 8ff3e6b merged ok, testing candidate = c17d533

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/376
success: http://build.servo.org/builders/linux2/builds/375
success: http://build.servo.org/builders/android/builds/411
success: http://build.servo.org/builders/mac1/builds/592
success: http://build.servo.org/builders/mac2/builds/593

[bors-servo]

fast-forwarding master to auto = c17d533