Fix CFB decryption performance in JS fallback for ciphers other than AES

[larabr]

By calling the underlying cipher decryption function for one block at a time.
Before, the entire ciphertext was passed for decryption, but only a subset of it (equal to the block size) was actually processed in each iteration.
This resulted in a lot of extra work for e.g. Cast5 decryption, whose decryption function supported decrypting more than a block at once.

This issue affected non-AES ciphers (legacy), such as Cast5, in Node 18+ or in browser.

Fix #1668 .

[twiss]

I think part of the issue here is that some of the block ciphers encrypt only one block (which this code seems to expect), while some ciphers encrypts everything. It would be good to make that uniform, and if all block ciphers encrypt only one block, then this code doesn't need to be changed.

If we do want to change it here, we should also change the encrypt function (even though it's less likely to be used).

[larabr]

@twiss even if the block cipher were to decrypt only one block, passing the full ciphertext in input to them seems confusing. I'd leave more extensive changes to the underlying ciphers be done as part as #1654 .

The encryption function is fine as-is since blockc is always of IV length