Inline testing

[vladfaust]

I like the idea of unit testing right within a source file (inspired by Zig).

struct Point {
  x, y : Float64
  length() => (this.x ** 2 + this.y ** 2).sqrt()
}

test "Point" {
  test ".length" {
    assert(Point(3, 4).length() =~ *(5, 0.01))
  }
}