Allow array types

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Arrays are only allowed inside of structs and are laid out like sequential struct fields.

struct Vec4 {
  v: [float:4];
}

Gives:

thread 280733 panic: TODO TypeFmt.init() base_ty=reflection.BaseType.fb.BaseType.Array
/home/thesm/.zvm/master/lib/std/debug.zig:291:22: 0x30b555 in panicExtra__anon_19121 (flatc-zig)
    std.builtin.panic(msg, trace, ret_addr);
                     ^
/home/thesm/.zvm/master/lib/std/debug.zig:266:15: 0x2e9449 in panic__anon_17084 (flatc-zig)
    panicExtra(null, null, format, args);
              ^
/home/thesm/Downloads/flatbufferz/src/common.zig:19:20: 0x2c9318 in todo__anon_16220 (flatc-zig)
    std.debug.panic("TODO " ++ fmt, args);
                   ^
/home/thesm/Downloads/flatbufferz/src/codegen.zig:183:20: 0x2c8f54 in format__anon_16215 (flatc-zig)
        } else todo("TypeFmt.init() base_ty={}", .{base_ty});