use sha1 hashes instead of c-strings to make records fixed size.
because it's hard to find collisions in sha1 hashes, this works well
in practise. And dynamic memory allocation for the variadic size keys
is not needed anymore. space is also reduced due to most key strings being
more than 20 bytes long.
calculating sha1 should be fast enough to not make any more overhead
than dynamic memory allocation did.
