Close padding hole in Perl_OpDumpContext struct

[richardleach]

The pahole tool, on AMD64, shows that this commit converts:

struct Perl_OpDumpContext {
        I32                        level;                /*     0     4 */

        /* XXX 4 bytes hole, try to pack */

        UV                         bar;                  /*     8     8 */
        PerlIO *                   file;                 /*    16     8 */
        _Bool                      indent_needed;        /*    24     1 */

        /* size: 32, cachelines: 1, members: 4 */
        /* sum members: 21, holes: 1, sum holes: 4 */
        /* padding: 7 */
        /* last cacheline: 32 bytes */
};

to

struct Perl_OpDumpContext {
        PerlIO *                   file;                 /*     0     8 */
        UV                         bar;                  /*     8     8 */
        I32                        level;                /*    16     4 */
        _Bool                      indent_needed;        /*    20     1 */

        /* size: 24, cachelines: 1, members: 4 */
        /* padding: 3 */
        /* last cacheline: 24 bytes */
};

---

• This set of changes does not require a perldelta entry.

[leonerd]

Fails for g++:

dump.c:1875:13: error: designator order for field ‘Perl_OpDumpContext::bar’ does not match declaration order in ‘Perl_OpDumpContext’

It seems in C++, the order of the named initialisers has to match the order of the fields in the definition. Which seems suuuuuper-annoying as surely the entire point of named initialisers is to remove that interdependence.. Grr. :(