Figure out what to do with flexible array members

A number of C structures make use of a flexible array member:

struct sockaddr {
     sa_family_t     sa_family;      /* Address family */
     char            sa_data[];      /* Socket address */
 };

When mapped to Rust, most of these are a public field of type [T; 0]. We could instead map them to Rust unsized [c_char] fields.

The current pattern has a few advantages over using DSTs:

• size_of works to get you the base size
• You can offset_of the field to get the start of the array if you need it
• You can work with the type through references, they don't need to be constructed as fat pointers
• Rust's flexible array members aren't all that convenient to work with

I expect we'll probably want to stick with the current pattern, but it's worth double checking that there isn't something better for 1.0.

Start by locating the existing Rust definitions that map C flexible array members to public [T; 0] fields, then compare that pattern with unsized [c_char] fields and the listed size_of, offset_of, and reference-use cases. Done means reaching and documenting a decision for the 1.0 representation, with its ABI and usability tradeoffs justified.