json: allow stringifying type fields comptime

[Cloudef]

This lets one serialize @typeInfo as well.

const std = @import("std");

const Packed = packed struct {
    foo: u1,
    bar: u3,
};

pub fn main() !void {
    @setEvalBranchQuota(100_000);
    comptime var buf: std.BoundedArray(u8, 1024) = .{};
    comptime try std.json.stringify(@typeInfo(Packed), .{.whitespace = .indent_2}, buf.writer());
    std.debug.print("{s}\n", .{comptime buf.constSlice()[0..].*});
}

{
  "struct": {
    "layout": "packed",
    "backing_integer": "u4",
    "fields": [
      {
        "name": "foo",
        "type": "u1",
        "default_value": null,
        "is_comptime": false,
        "alignment": 0
      },
      {
        "name": "bar",
        "type": "u3",
        "default_value": null,
        "is_comptime": false,
        "alignment": 0
      }
    ],
    "decls": [],
    "is_tuple": false
  }
}

[tauoverpi]

Composite types (afaik) do not currently have a format suitable for use outside compile errors thus if you stringify an anon struct the result is then implementation (of the zig compiler) defined and may be broken at any time making this change fragile to changes in type name formatting.

[Cloudef]

I don't think this has to offer any stability guarantees, especially for anon types.