- Updated to inline-able IRQ API
- Improved robustness of functions
- Added memory barrier to prevent the compiler from moving code outside of
a critical section guarded by irq_disable() ... irq_restore()
- Reduced overhead of `irq_disable()`
- After clearing the global interrupt enable (GIE) bit, IRQs remain enabled
for up to one CPU cycle
- The previous implementation just added a nop to fill that cycle
- This implementation uses the cycle for masking the return value
- Reduced overhead of `irq_restore()`
- Now only one CPU cycle is needed
- `irq_disable()`, `irq_restore()`, and `irq_enable()` work now in constant time
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