Use self-descriptive cache with type tags (#20137)

This should make our cache format more future proof w.r.t lazy
deserialization and other features:
* This makes cache ~5% larger (data + meta)
* This makes interpreted performance ~5% worse (just the serialization
steps, *not* overall time)
* No visible performance impact when compiled (probably because all
extra indirections like `read_str()` -> `read_str_bare()` etc are C
calls).

Note that I now serialize various little arbitrary JSON blobs (like
plugin data etc) using fixed format, since with the tags we can do this.

Author: ilevkivskyi