This function was added to a public header in [1] probably
unintentionally since it's not used anywhere, exposes implementation
details, and isn't related to the goals of that pull request.

[1]: 56cc3e99b6b9ec004255280337f6b8353f5e5b06

And re-embed any strings that can now fit inside the slot they've been
moved to

A short (2 bytes) will cause unaligned struct accesses when strings are
used as a buffer to directly store binary data.

Just split the comment for struct's one and array's one.

(gdb) ptype/o struct RString
/* offset    |  size */  type = struct RString {
/*    0      |    16 */    struct RBasic {
/*    0      |     8 */        VALUE flags;
/*    8      |     8 */        const VALUE klass;

                               /* total size (bytes):   16 */
                           } basic;
/*   16      |    24 */    union {
/*                24 */        struct {
/*   16      |     8 */            long len;
/*   24      |     8 */            char *ptr;
/*   32      |     8 */            union {
/*                 8 */                long capa;
/*                 8 */                VALUE shared;

                                       /* total size (bytes):    8 */
                                   } aux;

                                   /* total size (bytes):   24 */
                               } heap;
/*                24 */        struct {
/*   16      |    24 */            char ary[24];

                                   /* total size (bytes):   24 */
                               } embed;
/* XXX  8-byte padding  */

                               /* total size (bytes):   24 */
                           } as;

                           /* total size (bytes):   40 */
                         }
(gdb)

This commit adds support for embedded strings with variable capacity and
uses Variable Width Allocation to allocate strings.