Bools and Enums in generic parametrizations

[ozra]

I miss the possibility of using Bools and Enums (instead of resorting to empty types for signature decoration) in generic parametrizations, would be well considered in tandem with #934.

With "generic parametrizations" I refer to what Crystal calls "type vars" and "free vars".

I often use booleans in C++ templated classes for costly verification code: allowing noobs, and prototyping concepts, using classes in a clean simple fashion without risk of errors, while allowing pros/performance critical code to use them at a finer granular level without integrity checks. In some cases there's just a small part of the class that might have three or more variations depending on requirement, and then enum is nice (NoBeltsCommonCalc| BeltsOnCommonCalc|BeltsOnAdaptiveCalc).

In embedded real-time coding this is a definite must, though I know that such targets are far from an aim of Crystal (though a lovely thought). With that parenthesis aside: it's obviously good for high-performance code on more capable architectures too.

Pseudo Crystal:

class Foo(T, UseCheck = true)
  @x : T = T.new(0)
  
  def do_stuff(x : T) : Nil
    if UseCheck
      if x > 9
        com.log_fatal "Serious breach of confidence in Foo#do_stuff dude!"
        x = T.new(0)  # hopefully save nuclear power plant from blowing up
      end
    end
    @x = x
  end
end

a = Foo(QU8_56).new
a.do_stuff QU8_56.new(10)  # Lucky me the system was speced to have power enough for safety checks - just a couple of hours of city-wide black out.

b = Foo(QU8_56, false).new  # I'll use it responsibly - no room for checks on this EMF-safe lo-fi arch!
a.do_stuff QU8_56.new(10)   # Not! Well, no need to resign: the world just blew up!