Add `FromBool` (etc.) `@type interface`s.

[ta0kira]

This would have been ugly before the addition of #self. The implementations for built-in types can just delegate to bool.toInt(), etc.

[ta0kira]

Actually, this would just add to the dependencies required when linking binaries that use built-in types, so I'm going to hold off on this for now.

[ta0kira]

FromInt could still be useful for creating sequences of values.

For example:

@type range<#x>
  #x requires AsInt
  #x defines FromInt
(#x,#x) -> (Order<#x>)

// ...

traverse (range<?>('a','z') -> Char c) {
  // ...
}