…N` (#2502)

`Salt::new` is documented to return an error when the slice is longer
than `Salt::MAX_LEN` (32), but the length check was dropped in #2295.
A longer salt now panics with an index out of bounds while copying into
the fixed-size buffer.

`Salt::try_from` and the constructors that call it with a caller-supplied
salt, such as `Pbkdf2Params::hmac_sha256` and
`ScryptParams::from_params_and_salt`, panic the same way instead of
returning the documented error. Decoding is not affected because
`Salt::decode_value` checks the length.

Restore the check and add tests for a 33-byte salt and a 32-byte salt.

Assisted-by: Claude Code:claude-opus-5-5

…#2504)

The `*Owned` variants can support decoding and encoding, where the
`*Ref` variants can only encode.

As requested in #1611, this adds some basic documentation for how to use
the crate, including the `DecodeRsa*`/`EncodeRsa*` impls added for this
crate's own types in #2504.

This release includes a number of constant-time fixes but notably also
enables `sha2` by default and therefore removes the feature.