clean up `logic.R`

currently

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

but, I guess should be

```r
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))`