Implement multi-module feature and bulk-memory feature (#271)

Refine wasm loader and aot loader
Fix potential issue of os_mmap/os_munmap
Update document

samples/multi-module/wasm-apps/mB.c

@@ -0,0 +1,16 @@
+__attribute__((import_module("mA")))
+__attribute__((import_name("A"))) extern int
+A();
+
+int
+B()
+{
+    return 11;
+}
+
+int
+call_A()
+{
+    return A();
+}
+