Replace explicit IDs with table look-ups

That's a first step towards merging symmetric version of different functions
diff --git a/include/mbedtls/ecjpake.h b/include/mbedtls/ecjpake.h
index 9cad2e0..2f1a8ab 100644
--- a/include/mbedtls/ecjpake.h
+++ b/include/mbedtls/ecjpake.h
@@ -31,7 +31,7 @@
 #endif
 
 typedef enum {
-    MBEDTLS_ECJPAKE_CLIENT,
+    MBEDTLS_ECJPAKE_CLIENT = 0,
     MBEDTLS_ECJPAKE_SERVER,
 } mbedtls_ecjpake_role;