Convert to module system

[BoltonBailey]

@SamuelSchlesinger WRT your comment here: Perhaps migrating to the Lean module system is something we could try? Hopefully it would make some refactoring changes much faster to recompile and IIUC it can lead to perf improvements even without refactoring code at all.

[SamuelSchlesinger]

This sounds great, lets do this. Are there any tradeoffs to the downside?

[BoltonBailey]

Main downsides I am aware of are that we can not depend in the future on non-module-system libraries, and you have to be aware of how the module system handles meta code if you want to write any https://lean-lang.org/doc/reference/latest/Source-Files-and-Modules/