To be able to use "t" (or "nil") as a column name, pgloader needs to be
able to generate lisp code where those symbols are available. It's
simple enough in that a Common Lisp package that doesn't :use :cl
fullfills the condition, so intern user symbols in a specially crafted
package that doesn't :use :cl.
Now, we still need to be able to run transformation code that is using
the :cl package symbols and the pgloader.transforms functions too. In
this commit we introduce a heuristic to pick symbols either as functions
from pgloader.transforms or anything else in pgloader.user-symbols.
And so that user code may use NIL too, we provide an override mechanism
to the intern-symbol heuristic and use it only when parsing user code,
not when producing Common Lisp code from the parsed load command.
