When "with_gui" option is set to false, type information can not be loaded

[tomoyanonymous]

related to #86 #88

Currently, if with_gui option is set to false, the codes which contain function call to make_probe can not be compiled because the type information for the make_probe is not loaded even if it is not actually called (for example, with --emit-mir and --emit-bytecode option).

I think we should introduce an external function declaration syntax like extern make_probe : (string)->(float)->float, to make the compilation process runtime-agnostic.