machine: move errors.New calls to globals

Calling errors.New in an error path causes a heap allocation at an
already unfortunate moment. It is more efficient to create these error
values in globals and return these constant globals. If these errors are
not used (because the related code was optimized out), the globals will
also be optimized out.

src/machine/uart.go

@@ -4,6 +4,8 @@ package machine
 
 import "errors"
 
+var errUARTBufferEmpty = errors.New("UART buffer empty")
+
 type UARTConfig struct {
 	BaudRate uint32
 	TX       Pin
@@ -60,7 +62,7 @@ func (uart UART) ReadByte() (byte, error) {
 	// check if RX buffer is empty
 	buf, ok := uart.Buffer.Get()
 	if !ok {
-		return 0, errors.New("Buffer empty")
+		return 0, errUARTBufferEmpty
 	}
 	return buf, nil
 }