Naming: Rename Common lean to something else. (What ??)

# Why?

After a while we'll notice that everything that is truly generic here will be removed from `Common.lean` to other repositories, leaving only stuff that's either BEq \alpha or BEq \beta here.

# What?
- [ ] Perhaps we should rename Common.lean (eventually) reflecting this.

Start by reading Common.lean and reviewing which definitions are specific to BEq α or BEq β versus truly generic. Decide on a replacement name that reflects the remaining scope; done means the rename is agreed and the issue's eventual file references can be updated consistently.

Written by the indexing model from the issue text.