MemoryError on Metro M0 Express

Re:
https://forums.adafruit.com/viewtopic.php?f=63&t=149682

Can recreate with following:
```python
Adafruit CircuitPython 3.1.2 on 2019-01-07; Adafruit Metro M0 Express with samd21g18
>>> import board, busio
>>> import adafruit_character_lcd.character_lcd_i2c as character_lcd
>>> i2c = busio.I2C(board.SCL, board.SDA)
>>> lcd = character_lcd.Character_LCD_I2C(i2c, 20, 4)
>>> lcd.message = "Hello\nCircuitPython"
Traceback (most recent call last):
File "", line 1, in
File "adafruit_character_lcd/character_lcd.py", line 338, in message
MemoryError: memory allocation failed, allocating 160 bytes
>>>
```

But seems to work on M4:
```python
Adafruit CircuitPython 3.1.2 on 2019-01-07; Adafruit Metro M4 Express with samd51j19
>>> import board, busio
>>> import adafruit_character_lcd.character_lcd_i2c as character_lcd
>>> i2c = busio.I2C(board.SCL, board.SDA)
>>> lcd = character_lcd.Character_LCD_I2C(i2c, 20, 4)
>>> lcd.message = "Hello\nCircuitPython"
>>>
```