SHA3 get_digest should be const

[mborland]

We should be able to differentiate the XOF and non-xof paths like:

template <bool Const = !is_xof>
    [[nodiscard("Digest is the function return value")]] BOOST_CRYPT_GPU_ENABLED_CONSTEXPR
    compat::enable_if_t<Const, compat::expected<return_type, state>> get_digest() const noexcept;

This also allows the get_digest with amounts to only impact the XOF paths