std.meta: have fieldNames() return a mutable pointer to array

[nektro]

function value is only computed at comptime so returning the address local is safe and this allows the result to for instance be sorted without an extra copy

[rohlem]

In my understanding, pointers to comptime stack variables can be "leaked"/returned at comptime,
but their memory should become const according to accepted #7396.

[nektro]

I was doing the sorting also at comptime but this patch is perhaps the wrong way to go about it since if the type becomes runtime it becomes more likely to cause a segfault