Split up `hooks.cs` into individual `FhModule`s

[EvelynTSMG]

Maintenance

Please.

[EvelynTSMG]

Use FhModuleHandle to communicate easily between modules:

Guide:

• Setup fields: save_impl.cs:60-63.
• Construct: save_impl.cs:83-84.
• Get the module out: save_impl.cs:93-94.
  • Put this before init's return statement if you wish it to be an optional dependency.
• Use: save_impl.cs:116.