IsBool should handle v:true and v:false

[flwyd]

maktaba#ensure#IsBool(v:true) currently throws a WrongType exception because the type of v:true and v:false is boolean rather than number. It seems to me that boolean values should be handled as well.

Additionally, the absence of maktaba#value#IsBool seems like a notable omission.

[dbarnett]

Fair point, might as well accommodate those (and then bump maktaba's version so you could reliably depend on it).