Add a way to use comparison primitives

[Ailrun]

Is your feature request related to a problem? Please describe.
Currently, it is not possible to use comparison primitives easily. This issue is to discuss ways in which it would be nicer to use, implement, etc.

Describe the solution you'd like
In order to use comparison primitives, this language needs at least one of the following.

• Primitive pattern matches
• Integer2Boolean conversion
• Primitive if primIf (which uses an integer as its condition)

For each of these, implementation of higher level if (n <? 3) then t else f changes a lot.

• Primitive pattern matches
  The expression would be match n < 3 with 1 -> t; 0 -> f;.
• Integer2Boolean conversion
  It would be match integer2Boolean (n < 3) with C{1;0} -> t; C{2;0} -> f;
• Primitive if primIf
  It would be primIf (n < 3) t f

[Ailrun]

I think the language needs both primitive pattern matches and the integer2Boolean conversion.
However, the reasons for them are unrelated with each others. For primitive pattern matches, the reason is because it is more preferable to use pattern matching than multi-way ifs, since it should require at most one table-jump, and no more comparison. For the other one, it is the one required to implement if.

[Ailrun]

FYI: The reason why I think Integer2Boolean is better is that the way makes implementation more consistent. By 'implementation', I mean the ones for if for primitive conditions (like >?, >=?, ...) and for non-primitives (like is_even, is_in, ...). If I implement primIf, then two cases would use different underlying syntaxes after desugaring. (One uses primIf, and the other uses match)

[Ailrun]

Or maybe... adding boolean2primInt is better idea.
It still makes the impl. of the two cases consistent.