hash keys: validate hash key length at compile time

hash keys: validate hash key length at compile time

This was new to me, I hadn't encountered this failure while working
on the original "other I32 bugs".

The original test here was failing with an "Out of memory" error
since the long hash key length was overflowing the I32.

Once that was fixed the test was failing purely due to the invalid
code, once that was fixed the test passed so I removed the TODO.