Currently the task of freeing an instance is given to the crypto
template.  However, it has no type information on the instance so
we have to resort to checking type information at runtime.

This patch introduces a free function to crypto_type that will be
used to free an instance.  This can then be used to free an instance
in a type-safe manner.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>