riscv: add CSR-defining macros

[rmsyn]

Adds helper macros for defining CSR types:

• read-write:
  • have both getter and setter methods
  • able to read and write the CSR
• read-only:
  • only has getter methods
  • able to read the CSR
• write-only:
  • only has setter methods
  • able to write the CSR

Related to: https://github.com/rust-embedded/riscv/issues/218

[romancardenas]

Thanks! I'll review it ASAP

[rmsyn]

Using custom types (mainly enums) for setters and getters.

We could add another branch that takes an $var_ty: ident, [$($variant:ident, $variant_val:literal),*]$(,). That would allow to define a custom enum type, and use that enum in the getters/setters.

I can try to work something out.

[rmsyn]

I can try to work something out.

I added the enum branches, and an extra helper macro.

Let me know what you think.

[romancardenas]

@rmsyn I'm on it, but macros are difficult to review... 😥

Looks great. The only thing now is that I prefer to leave some macros out of the equation, and then let us call the proper sequence of macros according to the CSR. I think this will help us in the future to deal with macros (modular vs monolithic).

Regarding from and into: if we can get help from From and TryFrom traits, I'd go for them instead of maintaining repetitive methods.

Great job!! I'll keep studying all your code 😁