This is a step towards separating the mutable state on principals from const
state which can be made threadsafe. The remaining mutable fields will either
need to be moved off of nsIPrincipal or made threadsafe to destroy (e.g. using
nsMainThreadPtrHandle) and stored behind a mutex.

Serialization is handled for most types using a separate Deserializer class,
like how it was handled with `nsIURI` mutators. SystemPrincipal wasn't changed
as the `Read` method is a no-op for that class.

Differential Revision: https://phabricator.services.mozilla.com/D115092