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:

givenIf[Int] =newIf[Int]{defIf[T](cond: =>Int)(branch: =>T) =if (cond !=0) branch defIfElse[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:

givenIf[igmCond] =newIf[igmCond]{defIf[Textends igmCode](cond: => igmCond)(b: =>T) = igmIfBuild(cond, b) defIfElse[Textends igmCode, Fextends 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 singleConversion[A, B]andIf[B], convert to B.