feat(shell): add options --cpp and --discard
feat(frontends/lean): allow variables anywhere
feat(frontends/lean): namespaces also define scope for variables
refactor(library): remove some unnecessary sections
fix(doc/lean/test_single): "race condition" when running tests in parallel
fix(frontends/lean/elaborator): bug when reporting error position
feat(frontends/lean/parser): reject ambiguous parameter declaration, closes #242