[elixir] Outline sublist conditions, trivial passing

Given the two inputs, we should be branching our sublist checking based
on the length of the given lists. Even this barebones, lack of checking
implementatio passes 16 out of 19 tests.

[elixir] Get one more sublist test passing
[elixir] Simplify default case statement syntax

Here the underscore will match any tuple. There isn't any other length
tuple option besides equality if we've already handled la<lb and la>lb.
[elixir] Implement real sublist check

The readability is a little less than ideal with the two functions and
their various pattern matching clauses, but I think it's a relatively
elegant solution.