[Snek] [keith-packard/snek] 84ac99: Add 'abs' function to core builtins

[Keith Packard]

  Branch: refs/heads/master
  Home:   https://github.com/keith-packard/snek
  Commit: 84ac993f2c499328c58217ea07dd49387c1470f5
      https://github.com/keith-packard/snek/commit/84ac993f2c499328c58217ea07dd49387c1470f5
  Author: Keith Packard <keithp at keithp.com>
  Date:   2020-01-28 (Tue, 28 Jan 2020)

  Changed paths:
    M snek-base.builtin
    M snek-builtin.c

  Log Message:
  -----------
  Add 'abs' function to core builtins

This seems at least as important as math.sqrt?

Signed-off-by: Keith Packard <keithp at keithp.com>