Get rid of BoolConst

[adamjstewart]

The purpose of this PR is to remove BoolConst from the AST. This allows for a uniform treatment of "true" and "false" in parsing and evaluating. The following expressions:

cat.
cat :- true.

should have the same representation in the AST now.

[ZaidQureshi]

I know I sort of made a big deal out of this last night, but I was thinking and I don't think we need to do this as long as we can justify the way we do it in the master branch. The justification that we have for what we have in the master branch is that by having the two ways represented slightly differently we can evaluate the following stuff the same way swi-prolog does it.
db:
a(true).
b(X) :- a(X).

?- b(a(true)).
false.

With making them both the same thing, the evaluator would evaluate that to true.

Also, it is going to be a pain to fix all the code and tests and I would rather focus on the report.

[ZaidQureshi]

Fixed all evaluator and all tests to remove boolconst. although since you made the pr request you should have fixed all the code, like i did for the previous pr request but its ok.

[adamjstewart]

When I run make test I see the following warning message:

File "src/evaluator.ml", line 250, characters 34-63:
Warning 11: this match case is unused.

It looks like this block is duplicated. Or should one of them be for false?

[ZaidQureshi]

fixed

[coveralls]

Coverage increased (+0.09%) to 92.739% when pulling 08e7a66 on features/bool into 1d58fa3 on master.

[coveralls]

Coverage decreased (-0.04%) to 92.612% when pulling 08e7a66 on features/bool into 1d58fa3 on master.