optimize in for low memory device

[primstar-cool]

• add step OnSkipFunctionBodyExpr when read with skip_function_bodies
• add virtual op code OpCodeRaw, when read with skip_function_bodies, the virtual struct will save byte code in a uint8 array, and will write directly on output
• add macro wabt_expr_make_unique, for override operator new
• add MemoryAllocStream can output byte code in an allocated buffer