more tests, including tests for mutation of addition
mutation support for add/sub
error if mulitply use used in mutate mode
test new modInt api: return an int
modInt can and should just return an int

the modulus is always smaller than the divisor, so if the divisor is only 32bit, then the modulus is too
add tests for square(a), realized i didn't know how to count, rewrote multiply in terms of shift and add
why do we bother teaching kids long division and times tables when all they need to understand is bitwise operations?
document the api so far