Nix refactoring

[waltermoreira]

Modularize Nix flakes, using Flake Parts.

Split the components into the following flakes and dependencies:

                              
     Toolchain         Cache 
     │ │                  │  
     │ │                  │  
     │ │                  │  
     │ └──► Sequencelib ◄─┘  
     │             │         
     ▼             │         
 Symbolic          ▼         
 Server   ────► Interactive  
    │              │         
    │              ▼         
    └───────► AIProver       
                             

This work is happenning mostly in the branch cache-tool of Sequencelib.