Enable multi-module support for wasm-c-api (#426)

it is allowed that all imported functions and globals can be
linked by multi-module feature automatically or by wasm-c-api manually

samples/wasm-c-api/src/utils/multi_module_utils.c

@@ -0,0 +1,47 @@
+/*
+ * Copyright (C) 2019 Intel Corporation.  All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ */
+
+#include "bh_read_file.h"
+#include "wasm_export.h"
+
+static char *
+build_module_path(const char *module_name)
+{
+    const char *module_search_path = ".";
+    const char *format = "%s/%s.wasm";
+    int sz = strlen(module_search_path) + strlen("/") + strlen(module_name)
+             + strlen(".wasm") + 1;
+    char *wasm_file_name = BH_MALLOC(sz);
+    if (!wasm_file_name) {
+        return NULL;
+    }
+
+    snprintf(wasm_file_name, sz, format, module_search_path, module_name);
+    return wasm_file_name;
+}
+
+bool
+reader(const char *module_name, uint8 **p_buffer, uint32 *p_size)
+{
+    char *wasm_file_path = build_module_path(module_name);
+    if (!wasm_file_path) {
+        return false;
+    }
+
+    *p_buffer = (uint8_t *)bh_read_file_to_buffer(wasm_file_path, p_size);
+    BH_FREE(wasm_file_path);
+    return *p_buffer != NULL;
+}
+
+void
+destroyer(uint8 *buffer, uint32 size)
+{
+    if (!buffer) {
+        return;
+    }
+
+    BH_FREE(buffer);
+    buffer = NULL;
+}