Yet another implementation of the single aliases / multi aliases idea.

The new implementation is much simpler: single aliases are used everywhere
in the disambiguation phase; however, when a term needs to be looked for in
the library, it can be looked in a user-provided multi aliases environment instead.

NOTE: the new implementation fixes a bug of the previous implementation: the
most recent alias in the multi-alias set was printed last in the .moo files,
changing the performances of the system.