`math:prime?` thinks all non-integers are prime

```
(import std.Math)
(math:prime? 1.5) => true
```

This looks to be because the check rightly finds no integer divisors of the input.

`(math:divs 1.5)` likewise returns `[1.5]` where I might've expected an error, though given that a non-integer is not valid input in this case I'm unsure if matters.