fix(frontends/lean/elaborator): remove annotation left-over
fix(util/lazy_list): bug in filter operation
fix(frontends/lean/placeholder_elaborator): do not truncate stream of
solutions during class-instance resolution, closes #183

For example, in theorem inverse_unique at category.lean, implicit
arguments are synthesized for inverse_compose. The first solution H' is
not good, and produces a type incorrect solution
fix(frontends/lean/server): std::unordered_map::insert does replace
existing entry, fixes #188