Deserialization of comptime structs

This is a tracking issue for the following problem:

The implementation of structure deserialization uses a reference to `@field`, which is a runtime expression.

If a structure is declared `const` in a test, it will be `comptime` and this causes a problem where a reference to a `comptime` object is taken at runtime, which is not possible.

Reproduction of the problem:

```zig
const std = @import("std");
const builtin = @import("builtin");

pub fn main() !void {
const mystruc = .{ .age = @as(u8, 5), .name = @as([]const u8, "roger") };
const tinfo = @typeInfo(@TypeOf(mystruc));
inline for (tinfo.Struct.fields) |field| {
const x = &@field(mystruc, field.name);
std.debug.print("field = {any}\n", .{x});
}
}
```

output:

```
prog.zig:11:45: error: runtime value contains reference to comptime var
/tmp/tmpdir_tg2Kt7nRRXakarUS/tmp_kfQvMGZT1vr00sIzprog.zig:11:45: note: comptime var pointers are not available at runtime
```

**Update**

Note that dereferencing the pointer works:

```zig
const std = @import("std");
const builtin = @import("builtin");

pub fn main() !void {
const mystruc = .{ .age = @as(u8, 5), .name = @as([]const u8, "roger") };
const tinfo = @typeInfo(@TypeOf(mystruc));
inline for (tinfo.Struct.fields) |field| {
const x = &@field(mystruc, field.name);
std.debug.print("field = {any}\n", .{x.*});
}
}
```

output:

```
field = 5
field = { 114, 111, 103, 101, 114 }
```

which is normal, since there no longer is a reference during the `comptime` evaluation. So the workaround would be to somehow not pass a pointer in that case.