Rename Bool -> Truth

[cdsmith]

The Bool type is programming jargon (based on the last name of a logician). I believe it should be renamed to Truth instead, which will be more obvious.

[cdsmith]

I find myself wondering how possible this is. If we defined a different type instead of Bool, and then reimplemented ifThenElse from the RebindableSyntax extension, would this apply to guards and pattern matching as well?

If not, we could achieve essentially the same thing using the same approach as Program: define a type synonym, and then postprocess the compiler output to replace all occurrences of Bool with Truth.

[cdsmith]

This had to be done with the postprocessing trick, since RebindableSyntax still expects (==) to return Bool for pattern matching.