[Feature] If-statement

[cympfh]

To implement (type-safe) If-statement, the followings may need.

• Type inferred functions.
  • Functions have types of arguments, but don't know the type of output.
  • To investigate the type without concrete argument values, type-infer needs instead of eval.
    • Since latest cumin doesn't have loop or recursion, this is trivial.

fn f(x: Int) = [x]; // inferred as `Int -> Array<Int>`

// OK
fn g(x: Int) = { // inferred as `Int -> Array<Int>`
  if x == 0 {
    []
  } else {
    f(x)
  }
};

// NG
fn h(x: Int) = {
  if x == 0 {
    0
  } else {
    f(x) // this doesn't match to `0`.
  }
};

// NG
fn i(x: Int) = {
  if true {
    0
  } else {
    [x]
  }  // this is always (0: Int), but this is invalid.
};