Generalise formalisation of static auction (single good and combinatorial)

[clange]

1. finish formalising CombinatorialVickreyAuction.thy and its dependencies
2. revisit the general code for combinatorial static auctions; make sure that it sufficiently abstracts from the specific nVCG situation.
3. revisit the general code for single-good static auctions
   • as a first step, remove sga_ prefix from names
4. make (2) and (3) special cases of a more general "static auction" definition