Add an unsafe fn `SCB::steal`#169

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jonas-schievink wants to merge 2 commits intorust-embedded:masterfrom

jonas-schievink:scb-steal

Commits on Oct 1, 2019

Add an unsafe fn `SCB::steal`
jonas-schievink
committed

Commits on Oct 4, 2019

Clarify safety constraints
jonas-schievink
committed