Rename serial read_all to read_exact

This better reflects what that function is doing, namely reading as many
bytes as there is space in the given buffer.

examples/serial_dma.rs

@@ -44,7 +44,7 @@ fn main() -> ! {
 
     // start separate DMAs for sending and receiving the data
     let sending = tx.write_all(Pin::new(tx_buf), tx_channel);
-    let receiving = rx.read_all(Pin::new(rx_buf), rx_channel);
+    let receiving = rx.read_exact(Pin::new(rx_buf), rx_channel);
 
     // block until all data was transmitted and received
     let (mut tx_buf, tx_channel, tx) = sending.wait();
@@ -56,7 +56,7 @@ fn main() -> ! {
     tx_buf.copy_from_slice(b"hi again!");
 
     let sending = tx.write_all(tx_buf, tx_channel);
-    let receiving = rx.read_all(rx_buf, rx_channel);
+    let receiving = rx.read_exact(rx_buf, rx_channel);
 
     let (tx_buf, ..) = sending.wait();
     let (rx_buf, ..) = receiving.wait();

src/serial.rs

@@ -374,7 +374,7 @@ macro_rules! hal {
             #[cfg(feature = "stm32f303")]
             impl Rx<$USARTX> {
                 /// Fill the buffer with received data using DMA.
-                pub fn read_all<B, C>(
+                pub fn read_exact<B, C>(
                     self,
                     mut buffer: Pin<B>,
                     mut channel: C