error+runtime: hide r5js.NotAProcedureError behind a factory function.
error+runtime+misc: hide r5js.ParseError behind a factory function.
error+runtime: remove provide of no longer extant r5js.PrimitiveProcedureError.
error+runtime: hide r5js.QuasiquoteError behind a factory function.
error+runtime: hide r5js.ReadError behind a factory function.
error+scan: hide r5js.ScanError behind a factory function.
error+runtime: hide r5js.UnimplementedOptionError behind a factory function.
error: introduce r5js.Error.Type enum, but don't use yet.
error+runtime+test: turn r5js.Error into a struct, using type tags.