riscv: Use riscv_pac::CoreInterrupt in mie register

[romancardenas]

This PR adds new methods and functions for the mie CSR. The idea is to use the riscv_pac::CoreInterruptNumber trait to allow enabling and disabling target-specific interrupt sources (i.e., extending our API to non-standard interrupt sources).

Closes #314

[rmsyn]

Some suggestions for code re-use, otherwise LGTM.

[romancardenas]

Ooops! I pushed a few more changes. Can you please review the PR again? I will address your comments after your new review!

[rmsyn]

Small change requests for code-reuse. Otherwise, LGTM.

[romancardenas]

Before merging, I have some doubts about the naming:

What do you think about having fn riscv::register::mie::enable<I: CoreInterruptNumber>() and fn riscv::interrupt::enable_interrupt<I: CoreInterruptNumber>()`? Or do you prefer both functions to have the same name?

Additionally, should I include riscv::interrupt::is_interrupt_enabled<I>()? Maybe we don't want this kind of functions exposed in riscv::interrupt

[rmsyn]

What do you think about having fn riscv::register::mie::enable<I: CoreInterruptNumber>() and fn riscv::interrupt::enable_interrupt<I: CoreInterruptNumber>()`?

Either one is fine, I don't have strong opinions either way. enable is shorter, enable_interrupt is more explicit (though maybe redundant).

Or do you prefer both functions to have the same name?

Whatever name you choose, I would personally prefer consistent naming. Up to you, though.

Additionally, should I include riscv::interrupt::is_interrupt_enabled? Maybe we don't want this kind of functions exposed in riscv::interrupt

I think it would be good to include it. The same type of user that would want the other free functions would probably also prefer this one to be available.

[romancardenas]

I changed it to mie::enable because I found mie::enable_interrupt repetitive (machine interrupt enable :: enable interrupt...). I also added the interrupt::is_interrupt_enabled function.

Another philosophical question... Should mie::enable be unsafe? We are enabling an interrupt source, yes, but interrupts are effectively active in mstatus with mstatus::set_mie(). Usual critical section implementations manipulate the status register, not the mie register.

We can either leave both mie::enable and mstatus::set_mie unsafe or assume that enabling interrupt sources in mie is safe and unsafety comes from mstatus.

In any case, if you prefer, we can discuss this in a future PR