SPI improvements / Clock passing update

1. Using `impl Into<Hertz>` instead of Hertz now to increase
   usability for users
2. Update for SPI API to increase usability

src/uart.rs

@@ -46,7 +46,7 @@ pub enum Error {
     BreakCondition,
 }
 
-#[derive(Copy, Clone, PartialEq)]
+#[derive(Debug, PartialEq, Copy, Clone)]
 pub enum Event {
     // Receiver FIFO interrupt enable. Generates interrupt
     // when FIFO is at least half full. Half full is defined as FIFO