std.builtin: allow user code to set specific panic handlers

[wrongnull]

This pr will allow users to set custom specific panic handlers. In current implementation it looks like follows:

pub fn panic(message: []const u8, stack_trace: ?*std.builtin.StackTrace, _: ?usize) noreturn {
    if (std.mem.eql(u8, message, "access of inactive union field")) {
        //handle access of inactive union field
    }
    //handle other cases in the same manner
}

With proposed approach it will be possible to write something like this

pub fn panicInactiveUnionField(active: anytype, wanted: @TypeOf(active)) noreturn {
   //handle this panic
}

right in root source file.

[Vexu]

Do you have a use case for overriding these? IMO they're more implementation details than language features.

[wrongnull]

@Vexu
easy way to split some certain cases of panic without piling up ifs could be useful for user.
For instance one can write specific information about panic into log file. Or do (almost) anything you want with such data.

[Vexu]

This can be reconsidered if someone has an actual use case.