Misc code cleanup, mostly for alignment with the Google style guide.

[emilyfertig]

• Use C++20 instead of C++17.
• Remove text substitution of umap for std::unordered_map etc.
• Remove using std::vector etc. from the global namespace and just use std::vector, etc. everywhere.
• Use pre-increment instead of post-increment by default.
• Use auto only for complicated types.
• Get rid of globals.hh entirely and include libraries as needed in each file. Hard-code std::mt19937 as PRNG since I don't think we'll need to vary this.