add header to serialization format. fixes #22888

[JeffBezanson]

From the doc strings:

| Offset | Description                                     |
|:-------|:------------------------------------------------|
|   0    | tag byte (0x37)                                 |
|   1-2  | signature bytes "JL"                            |
|   3    | protocol version                                |
|   4    | bits 0-1: endianness: 0 = little, 1 = big       |
|   4    | bits 2-3: platform: 0 = 32-bit, 1 = 64-bit      |
|   5-7  | reserved                                        |

A header is written by default if you call serialize(::IO, x), so e.g. files saved this way will have a header. No headers are written automatically if you use a SerializationState.

The platform flags refer to known platforms as of the specified protocol version. If a new platform arises that violates any relevant assumptions, it will need a new flag. There are plenty of bits available.