fix(emacs/lean-project): use better prompt message for lean-project-create
feat(emacs/lean-settings): enable linja '--keep-going' option

Fix #234
feat(quantifiers.lean): change exists_unique to a constructively stronger formulation

the previous formulation was constructively probably to weak to be useful
feat(lean-input.el): add some new notation
feat(library): add definition of subsingleton and some other minor changes
feat(category): split category.lean in different files; add more constructions and theorems about isos
feat(binary.lean): add helper theorem for associative functions