add option types to encore

[kikofernandez]

addition of option types to encore. there is a type constructor called Maybe t
which represent the option type. valid data constructors for option
types are Just t and Nothing. there are some examples in the file
maybeType.enc. a small example would be:

def test_catch_all(z: Maybe int): string
  match z with
  z => "catchall"
  Just 32 => "Error, general condition should have been catched
  first"

this is translated to c into a bunch of if-else statements. i mean a
bunch because the developer can match on nested option types of the form
Maybe (Maybe int).

restrictions:

• there is not match all clause in the match expression

[EliasC]

All my previous issues have been resolved, but I stumbled upon a weird thing:

def foo() : ()
  match Just 42 with
    42 => ()

This program throws the compiler into an infinite loop somewhere in the typechecker (use the -voption to see how far compilation gets). Upon further inspection it seems like this happens when you put anything that isn't a variable access or of Maybe type as a pattern (I discovered this when I used a type cast). When this is fixed I think this PR is ready to be merged!

[EliasC]

This case looks suspicious (re: infinite loop)

[albertnetymk]

While we are on this, why is word preliminary used, instead of dependency?

[kikofernandez]

@albertnetymk I think you are in the wrong PR?

[albertnetymk]

Sorry, wrong tab.