Enable clock by default for stm32l0

Modify init function to return a Clock instance defined by a per-chip
SystemClock type and use this in macro setup

A proof of concept implementation for STM32 L0 chips.

This allows using embassy::main macros for STM32 devices that have the
clock setup logic.

embassy-stm32/src/lib.rs

@@ -36,6 +36,7 @@ pub mod time;
 
 pub use embassy_macros::interrupt;
 pub use pac::{interrupt, peripherals, Peripherals};
+pub use rcc::SystemClock;
 
 // workaround for svd2rust-generated code using `use crate::generic::*;`
 pub(crate) use pac::regs::generic;
@@ -45,6 +46,12 @@ pub struct Config {
     rcc: rcc::Config,
 }
 
+impl Config {
+    pub fn new(rcc: rcc::Config) -> Self {
+        Self { rcc }
+    }
+}
+
 impl Default for Config {
     fn default() -> Self {
         Self {
@@ -54,14 +61,12 @@ impl Default for Config {
 }
 
 /// Initialize embassy.
-pub fn init(config: Config) -> Peripherals {
+pub fn init(config: Config) -> (Peripherals, SystemClock) {
     let p = Peripherals::take();
 
     unsafe {
         dma::init();
         pac::init_exti();
-        rcc::init(config.rcc);
+        (p, rcc::init(config.rcc))
     }
-
-    p
 }