Splitting the LinearOrder instance on Nat

Stream: mathlib4

[Jeremy Tan]

In branch#nattwist I split the LinearOrder instance on Nat into its own file, splitting two files which partially depend on it and adjusting the imports.

It builds now, but is it a good idea? Is it wanted?

[Yaël Dillies]

This was already done and reverted by @Ruben Van de Velde, so probably "no"

[Jeremy Tan]

There are still two TODOs in master referring to #13092

[Jeremy Tan]

Which was why I did this shuffle privately in the first place

[Jeremy Tan]

https://github.com/leanprover-community/mathlib4/blob/06d443eab23c40af08453ebbcf97bd546f793a46/Mathlib/Data/Nat/Defs.lean#L62