Scope reduction for some functions

Some functions are [called only by a single source file](https://github.com/ChrisDodd/btyacc/blob/7b61a9cc72cdd00918a4876915cf5c258fd0256a/closure.c#L185).
Would you like to [mark them as "`static`"](https://en.wikipedia.org/wiki/Static_%28keyword%29) (and delete declarations from the header file "defs.h")?
