Fix code generation for nested unions.

[detly]

When a union is nested inside another union, it must be enclosed in a struct so that its "which" member is not overlapping with its other members.

Schema in question:

struct A {
    union {
        zero @0 :Void;
        one :union {
            first @1 :Void;
            second @2 :UInt16;
        }
    }
}

C code showing bug:

struct A {
	enum A_which which;
	capnp_nowarn union {
		enum A_one_which one_which;
		capnp_nowarn union {
			uint16_t second;
		} one;
	};
};

Correct C code:

struct A {
	enum A_which which;
	capnp_nowarn union {
		capnp_nowarn struct {
			enum A_one_which which;
			capnp_nowarn union {
				uint16_t second;
			};
		} one;
	};
};

Fixes #43.