Implement MIN and MAX functions

[Rangi42]

Fixes #723

[Rangi42]

This could be more relevant even when/if user-defined functions are implemented, if rgblink also supported MIN and MAX.

[Rangi42]

User-defined functions (#201) probably won't be available for a long time, but when they are they'll make this easy to define yourself (as long as they support a ternary operator, recursion, and variable arguments):

DEF min(x) = x
DEF min(x, y) = x < y ? x : y
DEF min(x, y, ...) = min(min(x, y), ...)

(or just the DEF min(x, y) = x < y ? x : y line if you only need a simple two-arg min)

Until then, it's not really something that needs building into the language: just use IF/ELSE.