Cipher selection
val default : Ciphersuite.ciphersuite listdefault is a list of ciphersuites this library uses by default.
val supported : Ciphersuite.ciphersuite listsupported is a list of ciphersuites this library supports (larger than default).
val fs : Ciphersuite.ciphersuite listfs is a list of ciphersuites which provide forward secrecy (sublist of default).
val fs_of : Ciphersuite.ciphersuite list -> Ciphersuite.ciphersuite listfs_of ciphers selects all ciphersuites which provide forward secrecy from ciphers.