Don't register same var twice.

[maiha]

There is a problem that struct.var could be registered twice. The minimum example is

struct struct_foo { struct struct_bar* bar; };
struct struct_bar { int x; };
typedef struct struct_bar struct_bar;

In this case, visit_var_declaration is called twice about struct_bar.x on reference and definitioin. Then, our parser produces as following.

struct StructBar
  x : LibC::Int
  x : LibC::Int
end

This pr fixes it, and produces expected code.

struct StructBar
  x : LibC::Int
end

Thanks.

[asterite]

Looks good, thank you! :-)