embassy-stm32: Simplify time

- Remove unused `MilliSeconds`, `MicroSeconds`, and `NanoSeconds` types
- Remove `Bps`, `KiloHertz`, and `MegaHertz` types that were only used
for converting to `Hertz`
- Replace all instances of `impl Into<Hertz>` with `Hertz`
- Add `hz`, `khz`, and `mhz` methods to `Hertz`, as well as
free function shortcuts
- Remove `U32Ext` extension trait

examples/stm32l0/src/bin/lorawan.rs

@@ -11,7 +11,7 @@ use embassy_lora::LoraTimer;
 use embassy_stm32::exti::ExtiInput;
 use embassy_stm32::gpio::{Input, Level, Output, Pull, Speed};
 use embassy_stm32::rng::Rng;
-use embassy_stm32::time::U32Ext;
+use embassy_stm32::time::khz;
 use embassy_stm32::{spi, Peripherals};
 use lorawan::default_crypto::DefaultFactory as Crypto;
 use lorawan_device::async_device::{region, Device, JoinMode};
@@ -34,7 +34,7 @@ async fn main(_spawner: embassy::executor::Spawner, p: Peripherals) {
         p.PA6,
         p.DMA1_CH3,
         p.DMA1_CH2,
-        200_000.hz(),
+        khz(200),
         spi::Config::default(),
     );