Overflow issues that occur when *unchecked-math* is disabled

[athos]

As reported in #330 (review), when running tests with *unchecked-math* disabled, you'll see some tests fail due to overflow caused by the byte fn:

$ lein update-in : assoc :global-vars '^:replace {*warn-on-reflection* nil, *unchecked-math* nil}' -- test
...
Ran 372 tests containing 3832 assertions.
1 failures, 17 errors.