optimized 'bnf_of_typ' further w.r.t. dead variables

optimization of 'bnf_of_typ' if all variables are dead

improved accounting for dead variables when naming set functions (refines d71c2737ee21)

added explicit killing

more precise error message

reuse same parser for '(co)datatype(_new)' as for 'bnf_decl'

tuned message;

tuned proofs;

tuned whitespace;

unused;

clarified semantic completion: retain kind.full_name as official item name for history; misc tuning;

try explicit semantic completion before syntax completion;

more explicit Completion.Item.range, independently of caret;

tuned whitespace;

clarified stretch_point_range wrt. UTF-16 surrogates; tuned;

tuned;

some rendering of completion range; tuned;

clarified completion names; tuned signature;

NEWS and documentation, including correction of long-overseen "*"

regenerated

keep only identifiers public which are explicitly requested or demanded by dependencies

explicit option for "open" code generation

tuned

more fine-grain notion of export