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

core/iwasm/include/aot_export.h

@@ -39,6 +39,7 @@ typedef struct AOTCompOption{
     char *target_abi;
     char *target_cpu;
     char *cpu_features;
+    bool enable_bulk_memory;
     uint32_t opt_level;
     uint32_t size_level;
     uint32_t output_format;