Add integer-based interface supporting SCM work

[hbierlee]

Introduce an integer-based interface for adding linear constraints, which is also used to implement some of the encoding algorithms. Major features:

• Improve order and binary encoding of variables
• Stabilize uniform order (bdd/gt) constraints and binary constraint (rca from gt/bdd decomp)
  • Note: mixed encoding almost there, but not quite stable
• Basic modeling interface
• Single constant multiplication databases for binary encoding of terms

TODO's before merge:

• Clippy / PRINT_COUPLING problem?
• Add expect_test!
• Refactor: TestDB::solve(..) duplicates some stuff from check_complete
• Yet more tests currently commented out that still need fixing
• Documentation (currently sparse and basic, not checked for completeness)
  • Related: probably some pub -> pub(crate)
• Migrate ecm/*.dimacs into Cnf source rust code objects

Feature requests to be handled later

• Move cnf: Vec<Vec<Lit>> functions into Cnf
• Reorganize testing in model