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/mC.c

@@ -0,0 +1,25 @@
+__attribute__((import_module("mA")))
+__attribute__((import_name("A"))) extern int
+A();
+
+__attribute__((import_module("mB")))
+__attribute__((import_name("B"))) extern int
+B();
+
+int
+C()
+{
+    return 12;
+}
+
+int
+call_A()
+{
+    return A();
+}
+
+int
+call_B()
+{
+    return B();
+}