[RFC] one type per register block

[japaric]

Right now, when dealing with derived peripherals, svd2rust uses type aliases:

pub mod tim2 {
    pub struct RegisterBlock { .. }
}

pub use tim2::RegisterBlock as Tim2;
pub type Tim3 = Tim2;
pub type Tim4 = Tim2;
pub type Tim5 = Tim2;

Thus all both the base peripheral and the derived peripherals have the same
type.

I propose we change svd2rust to generate a different type for each peripheral.

pub mod tim2 {
    pub struct RegisterBlock { .. }
}

pub struct Tim2 { register_block: tim2::RegisterBlock }
impl Deref for Tim2 { type Target = tim2::RegisterBlock; .. }

pub struct Tim3 { register_block: tim2::RegisterBlock }
impl Deref for Tim3 { type Target = tim2::RegisterBlock; .. }

pub struct Tim4 { register_block: tim2::RegisterBlock }
impl Deref for Tim4 { type Target = tim2::RegisterBlock; .. }

pub struct Tim5 { register_block: tim2::RegisterBlock }
impl Deref for Tim5 { type Target = tim2::RegisterBlock; .. }

The rationale is that you may want to have API that works for one peripheral,
e.g. TIM2, but not for the others. You can only enforce that if you have
different types for the peripherals. Another similar use case is APIs that
depend on some specific combinations of peripherals: PWM1 = GPIOA + TIM2, PWM2 =
GPIOB + TIM3, etc.

[japaric]

Implementation in #66