Missing some of the newer builtins
Hi,
The PICO8_BUILTINS list is missing a few more recent functions 
(and therefore, incorrectly minifying them):
- b'**time**',
- b'**t**', 
- b'**lshr**', 
- b'**rotr**', 
- b'**rotl**', 
- b'**ceil**'

I was about to create a PR, but can see there's already one (#54) still open that seems to cover these and more?