Support unnamed/anonymous structs and unions

[cadmic]

Stuff like this is used all over in the OoT repo:

struct Foo {
  int a;
  union {
    int b;
    float c;
  };
  struct {
    int d;
    float e;
  };
};

Modern GCC supports this, so perhaps it's easier to patch in those changes (see the individual commits for the sources). Thanks to @Thar0 for the idea here.

This required changing the syntax, so I used https://github.com/pixel-stuck/andrew-bison to regenerate the parse tables.

[AngheloAlf]

I didn't look at the code too closely, but drmario still matches 👍