Replace RSA by an abstract asymmetric key structure.

This declares the new structure asymmetric_key in crypt.h but does not
define it. Instead, it is defined in crypt.c so that only the functions
in this file can access the members of the structure directly.

Functions which previously took an (openssl-specific) RSA pointer
are changed to receive a pointer to struct asymmetric_key instead,
removing the requirement to include an openssl header file before
crypt.h can be included.