clean up `logic.R`

[jmbarbone]

currently

is_true(c(2, 3, NA))
#> [1] NA NA NA

but, I guess should be

is_true(c(2, 3, NA))
[1] FALSE FALSE FALSE

other things:

• replace which() with !is.na(x) & x (or !is.na(x) & !x)
• simplify is_true() to is_true(to_boolean(x))
• simplify is_false() to is_false(to_boolean(x))