Implement hash functions

New header file crypto_struct.h. The main file crypto.sh declares
structures which are implementation-defined. These structures must be
defined in crypto_struct.h, which is included at the end so that the
structures can use types defined in crypto.h.

Implement psa_hash_start, psa_hash_update and psa_hash_final. This
should work for all hash algorithms supported by Mbed TLS, but has
only been smoke-tested for SHA-256, and only in the nominal case.
diff --git a/include/psa/crypto_struct.h b/include/psa/crypto_struct.h
new file mode 100644
index 0000000..6bd4ed2
--- /dev/null
+++ b/include/psa/crypto_struct.h
@@ -0,0 +1,96 @@
+/**
+ * \file psa/crypto_struct.h
+ *
+ * \brief PSA cryptography module: Mbed TLS structured type implementations
+ */
+/*
+ *  Copyright (C) 2018, ARM Limited, All Rights Reserved
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ *  This file is part of mbed TLS (https://tls.mbed.org)
+ */
+
+#ifndef PSA_CRYPTO_STRUCT_H
+#define PSA_CRYPTO_STRUCT_H
+
+/* Include the Mbed TLS configuration file, the way Mbed TLS does it
+ * in each of its header files. */
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "../mbedtls/config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
+#include "mbedtls/cipher.h"
+#include "mbedtls/cmac.h"
+#include "mbedtls/gcm.h"
+#include "mbedtls/md.h"
+#include "mbedtls/md2.h"
+#include "mbedtls/md4.h"
+#include "mbedtls/md5.h"
+#include "mbedtls/ripemd160.h"
+#include "mbedtls/sha1.h"
+#include "mbedtls/sha256.h"
+#include "mbedtls/sha512.h"
+
+struct psa_hash_operation_s
+{
+    psa_algorithm_t alg;
+    union
+    {
+#if defined(MBEDTLS_MD2_C)
+        mbedtls_md2_context md2;
+#endif
+#if defined(MBEDTLS_MD4_C)
+        mbedtls_md4_context md4;
+#endif
+#if defined(MBEDTLS_MD5_C)
+        mbedtls_md5_context md5;
+#endif
+#if defined(MBEDTLS_RIPEMD160_C)
+        mbedtls_ripemd160_context ripemd160;
+#endif
+#if defined(MBEDTLS_SHA1_C)
+        mbedtls_sha1_context sha1;
+#endif
+#if defined(MBEDTLS_SHA256_C)
+        mbedtls_sha256_context sha256;
+#endif
+#if defined(MBEDTLS_SHA512_C)
+        mbedtls_sha512_context sha512;
+#endif
+    } ctx;
+};
+
+struct psa_mac_operation_s
+{
+    psa_algorithm_t alg;
+    int key_set : 1;
+    int iv_required : 1;
+    int iv_set : 1;
+    int has_input : 1;
+    uint8_t mac_size;
+    union
+    {
+#if defined(MBEDTLS_MD_C)
+        mbedtls_md_context_t hmac;
+#endif
+#if defined(MBEDTLS_CMAC_C)
+        mbedtls_cipher_context_t cmac;
+#endif
+    } ctx;
+};
+
+#endif /* PSA_CRYPTO_STRUCT_H */