feat: allow using ciphers with no IV or the same IV for every encryption

[svandenakker]

This commit enables modes of operation that do not require an IV, such
as ECB. It also allows reusing of IVs. This is not recommended if it can
be avoided, because if the (first few blocks of) plaintext are the same,
the (first few blocks of) the ciphertext will be the same, thus
disclosing information. However, this can be useful to implement a form
of "searchable encryption", where one can search whether a string is
present without having to decrypt all ciphertexts first.

This commit also renames references to "salt" with "IV". A salt is some
random string added to a passphrase. Some algorithm (such as PBKDF2) is
then used to derive a key from this passphrase. This key is then used by
the block cipher. The salt makes sure the same passphrase does not
always result in the same key. An IV, on the other hand, is used by a
block cipher mode of operation (such as CBC) to ensure that the same
plaintext does not always encrypt to the same ciphertext. In this case,
it is clear "IV" was meant.

[svandenakker]

Turns out, this was a pretty half-baked idea. A new MR is coming that should be a bit better.