Think about reconceiving boot as a (LiftRules)=>LiftRules function

More thoughts in [this ML thread](https://groups.google.com/forum/#!topic/liftweb/-RQdaEkDCjM).

Basic idea is that boot goes from being a no-arg function that does mutable setup, to being a function that takes an immutable `LiftRules` parameter and updates its values to a final immutable `LiftRules` that is returned and then installed as the final rules state. State mutability would be captured in things like `FactoryMaker`s in this conception, while most values would simply be immutable.

The transition here would be interesting, and this may push this into being something we should do for a hypothetical Lift 4, potentially.

Start by reading the linked ML thread, then inspect the boot entry point and LiftRules implementation to understand how mutable setup is currently installed. The proposal mentions FactoryMakers and a final immutable LiftRules value, but it does not define specific files, tests, or acceptance criteria; completion would require an agreed design for a future Lift 4 transition.

Written by the indexing model from the issue text.