redefine lists.pl (or other modules used in the compiler)

[jfmc]

The compiler depends on some library modules like lists.pl, which are then frozen: users are not allowed to load their own definitions for those modules from a toplevel.

Possible solutions:

1. introduce more complex module namespaces (so that two lists modules can coexists)
2. rename lists to sys_lists and define another lists.pl module that reexports all sys_lists predicates (easier but LPdoc documentation may look weird).