Start a fledgling "use" function.

A "use" function is now defined in fexl.fxl, and test/a1.fxl calls it.  At this
point we have to pass in the "source_" symbols because they aren't defined
within fexl.fxl itself.  Eliminating this would require making those symbols
somehow relative to the parent file.  I'll work it out.  The important thing is
that modularity in Fexl has a very exact definition within the ordinary
paradigm of function evaluation.

Rename "standard_context" as "source_context".

Updated the grammar synopsis in parse.c.