[Snek] [keith-packard/snek] 6588b3: Re-work slice code. Include tests.

[Keith Packard]

  Branch: refs/heads/master
  Home:   https://github.com/keith-packard/snek
  Commit: 6588b36bc09c96ad5327b3f6cf103a50091d5c53
      https://github.com/keith-packard/snek/commit/6588b36bc09c96ad5327b3f6cf103a50091d5c53
  Author: Keith Packard <keithp at keithp.com>
  Date:   2019-02-17 (Sun, 17 Feb 2019)

  Changed paths:
    M snek-code.c
    M snek-list.c
    M snek-string.c
    M snek.h
    M test/Makefile
    A test/slice.py

  Log Message:
  -----------
  Re-work slice code. Include tests.

Started by creating an exhaustive set of tests for slices, then made
the slice structure contain only the values relevant for
execution. Finally cleaned up the implementation to make it agree with
python3.

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