Add oval and ovalfill functions; update rnd function

[troyjfarrell]

These commits update the API. The oval and ovalfill functions were added in PICO-8 0.2.1. The rnd(table) addition happened in PICO-8 0.2.0.

[Shoozza]

Hello and thanks for the pull request! 😄
However there are some issues/suggestions with the new api calls:

1. Resulting ellipse is different from resulting circ call:

   circ(5,5,5,2)
   -- vs
   oval(0,0,10,10,3)
   

   Maybe use a similar approach as in in api.circ

2. Swapping x1,y1 and x1,y1 will result in a different oval

   ovalfill(0,0,10,10,2)
   -- vs
   ovalfill(10,10,0,0,3)
   

   Probably solvable with fixing the previous issue.

3. Calling with true as a color will work in pico-8 but not with picolove, also calling with false will not use black but the last color:

   rectfill(0,0,128,128,3)  -- we want to see black lines
   color(2)
   oval(0,0,10,10)
   oval(0,0,10,10,true) -- color is 0 afterwards in pico-8
   color(2)
   oval(0,0,10,10,false) -- color is 0 afterwards in pico-8
   

   Maybe convert col to 0 if boolean.

4. _ellipse could be a local function

[troyjfarrell]

It seems to me that we have a larger problem. There are problems with the current implementation of circ and with my implementation of ellipse. Let's leave this PR open while I work out how to write some tests for this.