Permit a fixed field to be declared inside a readonly struct.

[gafter]

From #1502

Permit a fixed field to be declared inside a readonly struct.

[sweemer]

For me the primary motivation of allowing a fixed field inside of a readonly struct would be to make it more "ref readonly friendly" as described here.

Not sure if this is the right place to ask but is this the only way to make such structs ref readonly friendly, or might there be a better way?