Draw Circle and Ellipse

[boo13]

Please consider adding the ability to draw a circle and/or ellipse.

An existing Adafruit example -- Adafruit_CircuitPython_SSD1306 -- implements a draw_circle function that works for my case in the meantime.

# Helper function to draw a circle from a given position with a given radius
# This is an implementation of the midpoint circle algorithm,
# see https://en.wikipedia.org/wiki/Midpoint_circle_algorithm#C_example for details
def draw_circle(xpos0, ypos0, rad, col=1):
    x = rad - 1
    y = 0
    dx = 1
    dy = 1
    err = dx - (rad << 1)
    while x >= y:
        oled.pixel(xpos0 + x, ypos0 + y, col)
        oled.pixel(xpos0 + y, ypos0 + x, col)
        oled.pixel(xpos0 - y, ypos0 + x, col)
        oled.pixel(xpos0 - x, ypos0 + y, col)
        oled.pixel(xpos0 - x, ypos0 - y, col)
        oled.pixel(xpos0 - y, ypos0 - x, col)
        oled.pixel(xpos0 + y, ypos0 - x, col)
        oled.pixel(xpos0 + x, ypos0 - y, col)
        if err <= 0:
            y += 1
            err += dy
            dy += 2
        if err > 0:
            x -= 1
            dx += 2
            err += dx - (rad << 1)

[ladyada]

hiya, a pull request would be welcome!