Introduce MD handle type

As has been previously done for ciphersuites, this commit introduces
a zero-cost abstraction layer around the type

  mbedtls_md_info const *

whose valid values represent implementations of message digest algorithms.

Access to a particular digest implementation can be requested by name or
digest ID through the API mbedtls_md_info_from_xxx(), which either returns
a valid implementation or NULL, representing failure.

This commit replaces such uses of `mbedtls_md_info const *` by an abstract
type `mbedtls_md_handle_t` whose valid values represent digest implementations,
and which has a designated invalid value MBEDTLS_MD_INVALID_HANDLE.

The purpose of this abstraction layer is to pave the way for builds which
support precisely one digest algorithm. In this case, mbedtls_md_handle_t
can be implemented as a two-valued type, with one value representing the
invalid handle, and the unique valid value representing the unique enabled
digest.
diff --git a/tests/suites/test_suite_rsa.function b/tests/suites/test_suite_rsa.function
index f2a9b98..4d22c9b 100644
--- a/tests/suites/test_suite_rsa.function
+++ b/tests/suites/test_suite_rsa.function
@@ -497,8 +497,10 @@
     TEST_ASSERT( mbedtls_rsa_check_privkey( &ctx ) == 0 );
 
 
-    if( mbedtls_md_info_from_type( digest ) != NULL )
+    if( mbedtls_md_info_from_type( digest ) != MBEDTLS_MD_INVALID_HANDLE )
+    {
         TEST_ASSERT( mbedtls_md( mbedtls_md_info_from_type( digest ), message_str->x, message_str->len, hash_result ) == 0 );
+    }
 
     TEST_ASSERT( mbedtls_rsa_pkcs1_sign( &ctx, &rnd_pseudo_rand, &rnd_info,
                                          MBEDTLS_RSA_PRIVATE, digest, 0,
@@ -538,8 +540,10 @@
     TEST_ASSERT( mbedtls_rsa_check_pubkey( &ctx ) == 0 );
 
 
-    if( mbedtls_md_info_from_type( digest ) != NULL )
+    if( mbedtls_md_info_from_type( digest ) != MBEDTLS_MD_INVALID_HANDLE )
+    {
         TEST_ASSERT( mbedtls_md( mbedtls_md_info_from_type( digest ), message_str->x, message_str->len, hash_result ) == 0 );
+    }
 
     TEST_ASSERT( mbedtls_rsa_pkcs1_verify( &ctx, NULL, NULL, MBEDTLS_RSA_PUBLIC, digest, 0, hash_result, result_str->x ) == result );