move meta sections into new files

[chenson2018]

In #243 I left some meta sections within other files, specifically in Cslib/Foundations/Data/HasFresh.lean and Cslib/Foundations/Semantics/LTS/Basic.lean. I think it would be less confusing for imports and maybe more performant for these to be in their own files. There is also ReductionSystem, where everything is currently public and exposed.

(I'd like to double check my understand of meta imports first before making any changes.)