Issue 4858: Let define-session-public place variables natively into parser

Issue 4858: Let define-session-public place variables natively into parser

Putting them as native variables in the parser module (rather than
using export/import) makes `set!' and `define' equivalent rather than
having `define' create a shadowing definition of the session variable.
That is important in order to avoid the values of the variable
diverging between parser module and `lily'.