[Routing] serialize the compiled route to speed things up

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | not really
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12012, #12220
| License       | MIT
| Doc PR        | -

This also makes the CompiledRoute implement Serializable in order to:

1. make the serialization format shorter
2. have no null bytes in there, which the native serializer add for private properties, and thus would complicate saving in databases etc.
3.  Since the Route now includes the CompiledRoute in the serialization, the CompiledRoute serialization must be consistent as well. We can only ensure that in future symfony version by implementing Serializable.

We should add to our symfony BC promise, that only classes that implement Serializable are ensured to be deserializable correctly with serialized representations of the class in previous symfony versions.
Can you use underscore names as this is what we are using everywhere else for array keys?