feat(library/data/bool): do not use `!` as notation for bnot, rename band/bor -> and/or
feat(frontends/lean): add `!` operator the "dual" of `@`, closes #220
refactor(library): rename namespace eq_ops to eq.ops
refactor(library/data/nat): rename to_nat to of_num
refactor(library/data/nat): use new operator '!'
feat(frontends/lean/elaborator): modify '!' semantics: it stops consuming arguments as soon it finds an argument that does not occur in the rest of the type.