fix(emacs/lean-syntax): syntax highlight for composite names in declarations
fix(frontends/lean/parser): add two kinds of no_undef_id behavior: to (global) constant; to local constant
fix(frontends/lean/parser): save identifier info for undef local
feat(library/tactic/exact_tactic): modify 'exact' tactic semantics, use higher-order unification

See new node.inj4 theorem, we need the extra power to be able to avoid type information at
    exact (assume e₁ e₂, e₁)
feat(emacs/lean-syntax): add 'eassumption' highlight
fix(frontends/lean): improve begin-end construct
fix(frontends/lean/elaborator): missing information when displaying unsolved placeholders