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_pkcs5.function b/tests/suites/test_suite_pkcs5.function
index 26f1d33..a8f0523 100644
--- a/tests/suites/test_suite_pkcs5.function
+++ b/tests/suites/test_suite_pkcs5.function
@@ -12,14 +12,14 @@
                   int it_cnt, int key_len, data_t * result_key_string )
 {
     mbedtls_md_context_t ctx;
-    const mbedtls_md_info_t *info;
+    mbedtls_md_handle_t info;
 
     unsigned char key[100];
 
     mbedtls_md_init( &ctx );
 
     info = mbedtls_md_info_from_type( hash );
-    TEST_ASSERT( info != NULL );
+    TEST_ASSERT( info != MBEDTLS_MD_INVALID_HANDLE );
     TEST_ASSERT( mbedtls_md_setup( &ctx, info, 1 ) == 0 );
     TEST_ASSERT( mbedtls_pkcs5_pbkdf2_hmac( &ctx, pw_str->x, pw_str->len, salt_str->x, salt_str->len,
                                      it_cnt, key_len, key ) == 0 );