Add structure representing TLS records

This commit adds a structure `mbedtls_record` whose instances
represent (D)TLS records. This structure will be used in the
subsequent adaptions of the record encryption and decryption
routines `ssl_decrypt_buf` and `ssl_encrypt_buf`, which currently
take the entire SSL context as input, but should only use the
record to be acted on as well as the record transformation to use.

include/mbedtls/ssl_internal.h

@@ -504,6 +504,42 @@ struct mbedtls_ssl_transform
 #endif
 };
 
+/*
+ * Internal representation of record frames
+ *
+ * The header layout is chosen to facilitate the computation of
+ * authentication tags which often use the header bytes laid out
+ * exactly as in the struct; note that it does not match what's
+ * transferred on the wire.
+ *
+ * Instances come in two flavors:
+ * (1) Encrypted
+ *     These always have data_offset = 0
+ * (2) Unencrypted
+ *     These have data_offset set to the length of the
+ *     fixed part of the IV used for encryption.
+ *
+ * The reason for the data_offset in the unencrypted case
+ * is to allow for in-place conversion of an unencrypted to
+ * an encrypted record. If the offset wasn't included, the
+ * encrypted content would need to be shifted afterwards to
+ * make space for the fixed IV.
+ *
+ */
+typedef struct
+{
+    uint8_t ctr[8];         /*!< Record sequence number        */
+    uint8_t type;           /*!< Record type                   */
+    uint8_t ver[2];         /*!< SSL/TLS version               */
+    uint8_t len[2];         /*!< Content length, little endian */
+
+    unsigned char *buf;     /*!< Memory buffer enclosing the record content */
+    size_t buf_len;         /*!< Buffer length */
+    size_t data_offset;     /*!< Offset of record content */
+    size_t data_len;        /*!< Length of record content */
+
+} mbedtls_record;
+
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
 /*
  * List of certificate + private key pairs