Add IRQ-driven buffered USART implementation for STM32 v2 usart (#356)

* Add IRQ-driven buffered USART implementation for STM32 v2 usart

* Implementation based on nRF UARTE, but simplified to not use DMA to
  avoid complex interaction between DMA and USART.
* Implementation of AsyncBufRead and AsyncWrite traits
* Some unit tests to ring buffer
* Update polyfill version
* Update sub module to get usart IRQ fix

examples/stm32l0/Cargo.toml

@@ -21,6 +21,7 @@ embassy = { version = "0.1.0", path = "../../embassy", features = ["defmt", "def
 embassy-traits = { version = "0.1.0", path = "../../embassy-traits", features = ["defmt"] }
 embassy-stm32 = { version = "0.1.0", path = "../../embassy-stm32", features = ["defmt", "defmt-trace", "stm32l072cz", "time-driver-tim3"]  }
 embassy-hal-common = {version = "0.1.0", path = "../../embassy-hal-common" }
+embassy-macros = { path = "../../embassy-macros" }
 
 defmt = "0.2.0"
 defmt-rtt = "0.2.0"

examples/stm32l0/src/bin/usart_irq.rs

@@ -0,0 +1,51 @@
+#![no_std]
+#![no_main]
+#![feature(trait_alias)]
+#![feature(type_alias_impl_trait)]
+#![allow(incomplete_features)]
+
+#[path = "../example_common.rs"]
+mod example_common;
+
+use example_common::*;
+
+use defmt::panic;
+use embassy::executor::Spawner;
+use embassy::io::{AsyncBufReadExt, AsyncWriteExt};
+use embassy_stm32::dma::NoDma;
+use embassy_stm32::interrupt;
+use embassy_stm32::usart::{BufferedUart, Config, State, Uart};
+use embassy_stm32::{rcc, Peripherals};
+
+#[embassy::main]
+async fn main(_spawner: Spawner, mut p: Peripherals) {
+    let mut rcc = rcc::Rcc::new(p.RCC);
+    rcc.enable_debug_wfe(&mut p.DBGMCU, true);
+
+    static mut TX_BUFFER: [u8; 8] = [0; 8];
+    static mut RX_BUFFER: [u8; 256] = [0; 256];
+
+    let mut config = Config::default();
+    config.baudrate = 9600;
+
+    let usart = Uart::new(p.USART1, p.PA10, p.PA9, NoDma, NoDma, config);
+    let mut state = State::new();
+    let mut usart = unsafe {
+        BufferedUart::new(
+            &mut state,
+            usart,
+            interrupt::take!(USART1),
+            &mut TX_BUFFER,
+            &mut RX_BUFFER,
+        )
+    };
+
+    usart.write_all(b"Hello Embassy World!\r\n").await.unwrap();
+    info!("wrote Hello, starting echo");
+
+    let mut buf = [0; 4];
+    loop {
+        usart.read_exact(&mut buf[..]).await.unwrap();
+        usart.write_all(&buf[..]).await.unwrap();
+    }
+}