Key derivation by small input steps: proof-of-concept

Document the new API. Keep the old one.

Implement for HKDF. Use it in a few test cases.

Key agreement is still unchanged.
diff --git a/include/psa/crypto_struct.h b/include/psa/crypto_struct.h
index ee3ecd7..bebc5c4 100644
--- a/include/psa/crypto_struct.h
+++ b/include/psa/crypto_struct.h
@@ -165,6 +165,8 @@
 #endif
     uint8_t offset_in_block;
     uint8_t block_number;
+    uint8_t state : 2;
+    uint8_t info_set : 1;
 } psa_hkdf_generator_t;
 #endif /* MBEDTLS_MD_C */