Add functions to read the type, mode, name and key_bitlen fields from
mbedtls_cipher_info_t. These are the fields that applications are most
likely to care about.
TLS code also uses iv_size and block_size, which it might make sense to
expose, but most applications shouldn't need those, so I'm not exposing them
for now.
Call the new functions in unit tests, so they're at least smoke-tested.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>