Add generic BlockInt trait and parse_block_int<T>() function to
cipher-core, eliminating duplicate parsing code in aes and des crates.
- BlockInt trait abstracts over u64/u128 integer types
- Supports hex (0x), binary (0b), and ASCII string formats
- Improved BlockError::InvalidByteStringLength with max/actual fields
- Implements `chunks()` returning iterator over 4-element subkey arrays.
- Implements `chunks_rev()` returning reverse iterator for decryption.
- Enables cipher rounds to iterate over round keys sequentially and in reverse.
- Implements `inv_shift_rows` performing cyclic right shifts on state rows.
- Implements `inv_mix_columns` using inverse Galois Field matrix multiplication.
- Adds unit tests verifying inverse transformations are true inverses.
- Implements `shift_rows` performing cyclic shifts on the state matrix rows.
- Implements `mix_columns` using Galois Field matrix multiplication.
- Adds `gmul` and `xtime` const helpers for GF(2^8) arithmetic.
- Adds unit tests verifying transformations against FIPS-197 vectors.