Make Peripherals::steal() zero cost

[Dirbaio]

Peripherals::steal() has a write to a static mut:

cortex-m/src/peripheral/mod.rs

Line 180 in 6b01313

TAKEN = true;

I expected it to be zero-cost, just constructing and returning the Peripherals which is a noop since it is ZST. This was a bit surprising.

[adamgreig]

cc #186