hal: masking APIs for individual IRQs

[hawkw]

this will probably help with IOAPIC (#28) and PIC drivers.

probably something like the existing Control::enable_irq/disable_irq, but for a specific vector

mycelium/hal-core/src/interrupt.rs

Lines 10 to 14 in ff48f57

	/// Disable interrupts.
	unsafe fn disable_irq(&mut self);
	/// Enable interrupts.
	unsafe fn enable_irq(&mut self);

we can then use this to implement a drop guard API, like enter_critical:

mycelium/hal-core/src/interrupt.rs

Lines 34 to 39 in ff48f57

	fn enter_critical(&mut self) -> CriticalGuard<'_, Self> {
	unsafe {
	self.disable_irq();
	}
	CriticalGuard { ctrl: self }
	}

(possibly enable_irq and disable_irq should be renamed to enable_all/disable_all or something — that naming incorrectly implies that they enable/disable a specific IRQ...)

[hawkw]

we might actually want the actual implementation of this to be in the Vector trait, with a ref to the interrupt controller; depending on the interrupt, the way it is masked might differ, for example, on x86, it could be a CPU interrupt, a PIC interrupt, or an IOAPIC interrupt...

[hawkw]

cc @iximeow

[hawkw]

did this for ISA IRQs in #490