Unnamed Structure and Union Fields

In ISO C11, it's legal and useful to define a struct like

```
struct {
int a;
union {
int b;
float c;
};
int d;
} foo;
```

in such a case, an instance foo of struct bar would have foo.b and foo.c accessing the same memory as different types. In addition, it is also legal and useful to do this:

```
struct foo {
int a;
};

struct bar {
struct foo;
float b;
char *c;
};
```

in which case, a pointer to bar can be easily and safely cast to a pointer to foo, and an instance baz of struct bar would have member baz.a as an int.

Millfork already supports both structure and union types, and I feel that this ability would make it even better.

The issue names no files or tests; start by locating Millfork's existing structure and union type handling. Done means supporting the two ISO C11-style unnamed structure and union forms, including the member access and embedding behavior described in the examples.

Written by the indexing model from the issue text.