refactor: remove unused buffer module from rustcrypto platform

The buffer module provided FixedPlainText and FixedCipherText wrapper types
but was not used by the current AES-CTR cipher implementation, which uses
simple [u8; 256] arrays directly. Removing it simplifies the crate and
follows YAGNI principles.

- Remove buffer module declaration from lib.rs
- Remove buffer type re-exports (BufferError, FixedCipherText, etc.)
- Delete unused buffer.rs file

All cipher tests continue to pass with this simplification.

Author: rusty1968

platform/impls/rustcrypto/src/lib.rs

@@ -10,13 +10,9 @@
 
 #![no_std]
 
-pub mod buffer;
 pub mod cipher;
 
 // Re-export commonly used ECDSA types
 
-// Re-export commonly used buffer types
-pub use buffer::{BufferError, FixedCipherText, FixedPlainText, DEFAULT_MAX_MESSAGE_SIZE};
-
 // Re-export commonly used cipher types
 pub use cipher::{Aes256CtrCipher, Aes256GcmCipher, RustCryptoCipherError};