Add an API for if with other types than native booleans

[RubenVerg]

Provide a typeclass called something like If, taking a single type parameter A: a condition of the type A would call the instance If[A].If or If[A].IfElse. This can be used both as a conversion to Boolean that only works in ifs (eg:

given If[Int] = new If[Int] {
  def If[T](cond: => Int)(branch: => T) = if (cond != 0) branch
  def IfElse[T, F](cond: => Int)(t: => T)(f: => F) = if (cond != 0) t else f

) or for a completely unrelated usage to standard if, eg in DSLs:

given If[igmCond] = new If[igmCond] {
  def If[T extends igmCode](cond: => igmCond)(b: => T) = igmIfBuild(cond, b)
  def IfElse[T extends igmCode, F extends igmCode](cond: => igmCond)(t: => T)(f: => F) = igmIfElseBuild(cond, t, f)
}

This implies that the if/else condition doesn't necessarily return one of T or F, since in this specific usage (the one that led me to the suggestion) igmIfBuild and igmIfElseBuild inspect branches to "flatten" if/else clauses into a single if/else if/else if/else/end clause (this is an internal to external DSL thing).

In the case the condition in the if (call it A) isn't Boolean and there is no If[A], try looking for a Conversion[A, Boolean], and only if it doesn't exist, and there's a single Conversion[A, B] and If[B], convert to B.